Introduce HTML Tag Processor

This commit pulls in the HTML Tag Processor from the Gutenbeg repository.
The Tag Processor attempts to be an HTML5-spec-compliant parser that
provides the ability in PHP to find specific HTML tags and then add,
remove, or update attributes on that tag. It provides a safe and reliable
way to modify the attribute on HTML tags.

```php
// Add missing `rel` attribute to links.
$p = new WP_HTML_Tag_Processor( $block_content );
if ( $p->next_tag( 'A' ) && empty( $p->get_attribute( 'rel' ) ) ) {
    $p->set_attribute( 'noopener nofollow' );
}
return $p->get_updated_html();
```

Introduced originally in WordPress/gutenberg#42485 and developed within
the Gutenberg repository, this HTML parsing system was built in order
to address a persistent need (properly modifying HTML tag attributes)
and was motivated after a sequence of block editor defects which stemmed
from mismatches between actual HTML code and expectectations for HTML
input running through existing naive string-search-based solutions.

The Tag Processor is intended to operate fast enough to avoid being an
obstacle on page render while using as little memory overhead as possible.
It is practically a zero-memory-overhead system, and only allocates memory
as changes to the input HTML document are enqueued, releasing that memory
when flushing those changes to the document, moving on to find the next
tag, or flushing its entire output via `get_updated_html()`.

Rigor has been taken to ensure that the Tag Processor will not be consfused
by unexpected or non-normative HTML input, including issues arising from
quoting, from different syntax rules within `<title>`, `<textarea>`, and
`<script>` tags, from the appearance of rare but legitimate comment and
XML-like regions, and from a variety of syntax abnormalities such as
unbalanced tags, incomplete syntax, and overlapping tags.

The Tag Processor is constrained to parsing an HTML document as a stream
of tokens. It will not build an HTML tree or generate a DOM representation
of a document. It is designed to start at the beginning of an HTML
document and linearly scan through it, potentially modifying that document
as it scans. It has no access to the markup inside or around tags and it
has no ability to determine which tag openers and tag closers belong to each
other, or determine the nesting depth of a given tag.

It includes a primitive bookmarking system to remember tags it has previously
visited. These bookmarks refer to specific tags, not to string offsets, and
continue to point to the same place in the document as edits are applied. By
asking the Tag Processor to seek to a given bookmark it's possible to back
up and continue processsing again content that has already been traversed.

Attribute values are sanitized with `esc_attr()` and rendered as double-quoted
attributes. On read they are unescaped and unquoted. Authors wishing to rely on
the Tag Processor therefore are free to pass around data as normal strings.

Convenience methods for adding and removing CSS class names exist in order to
remove the need to process the `class` attribute.

```php
// Update heading block class names
$p = new WP_HTML_Tag_Processor( $html );
while ( $p->next_tag() ) {
    switch ( $p->get_tag() ) {
	case 'H1':
	case 'H2':
	case 'H3':
	case 'H4':
	case 'H5':
	case 'H6':
	    $p->remove_class( 'wp-heading' );
	    $p->add_class( 'wp-block-heading' );
	    break;
}
return $p->get_updated_html();
```

The Tag Processor is intended to be a reliable low-level library for traversing
HTML documents and higher-level APIs are to be built upon it. Immediately, and
in Core Gutenberg blocks it is meant to replace HTML modification that currently
relies on RegExp patterns and simpler string replacements.

See the following for examples of such replacement:
    WordPress/gutenberg@1315784
    https://github.com/WordPress/gutenberg/pull/45469/files#diff-dcd9e1f9b87ca63efe9f1e834b4d3048778d3eca41aa39c636f8b16a5bb452d2L46
    WordPress/gutenberg#46625

Co-Authored-By: Adam Zielinski <[email protected]>
Co-Authored-By: Bernie Reiter <[email protected]>
Co-Authored-By: Grzegorz Ziolkowski <[email protected]>

src/wp-includes/class-wp-html-attribute-token.php

@@ -0,0 +1,93 @@
+<?php
+/**
+ * HTML Tag Processor: Attribute token structure class.
+ *
+ * @package WordPress
+ * @subpackage HTML
+ * @since 6.2.0
+ */
+
+if ( ! class_exists( 'WP_HTML_Attribute_Token' ) ) :
+
+/**
+ * Data structure for the attribute token that allows to drastically improve performance.
+ *
+ * This class is for internal usage of the WP_HTML_Tag_Processor class.
+ *
+ * @access private
+ * @since 6.2.0
+ *
+ * @see WP_HTML_Tag_Processor
+ */
+class WP_HTML_Attribute_Token {
+	/**
+	 * Attribute name.
+	 *
+	 * @since 6.2.0
+	 * @var string
+	 */
+	public $name;
+
+	/**
+	 * Attribute value.
+	 *
+	 * @since 6.2.0
+	 * @var int
+	 */
+	public $value_starts_at;
+
+	/**
+	 * How many bytes the value occupies in the input HTML.
+	 *
+	 * @since 6.2.0
+	 * @var int
+	 */
+	public $value_length;
+
+	/**
+	 * The string offset where the attribute name starts.
+	 *
+	 * @since 6.2.0
+	 * @var int
+	 */
+	public $start;
+
+	/**
+	 * The string offset after the attribute value or its name.
+	 *
+	 * @since 6.2.0
+	 * @var int
+	 */
+	public $end;
+
+	/**
+	 * Whether the attribute is a boolean attribute with value `true`.
+	 *
+	 * @since 6.2.0
+	 * @var bool
+	 */
+	public $is_true;
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 6.2.0
+	 *
+	 * @param string $name         Attribute name.
+	 * @param int    $value_start  Attribute value.
+	 * @param int    $value_length Number of bytes attribute value spans.
+	 * @param int    $start        The string offset where the attribute name starts.
+	 * @param int    $end          The string offset after the attribute value or its name.
+	 * @param bool   $is_true      Whether the attribute is a boolean attribute with true value.
+	 */
+	public function __construct( $name, $value_start, $value_length, $start, $end, $is_true ) {
+		$this->name            = $name;
+		$this->value_starts_at = $value_start;
+		$this->value_length    = $value_length;
+		$this->start           = $start;
+		$this->end             = $end;
+		$this->is_true         = $is_true;
+	}
+}
+
+endif;

src/wp-includes/class-wp-html-span.php

@@ -0,0 +1,56 @@
+<?php
+/**
+ * HTML Span: Represents a textual span inside an HTML document.
+ *
+ * @package WordPress
+ * @subpackage HTML
+ * @since 6.2.0
+ */
+
+if ( ! class_exists( 'WP_HTML_Span' ) ) :
+
+/**
+ * Represents a textual span inside an HTML document.
+ *
+ * This is a two-tuple in disguise, used to avoid the memory
+ * overhead involved in using an array for the same purpose.
+ *
+ * This class is for internal usage of the WP_HTML_Tag_Processor class.
+ *
+ * @access private
+ * @since 6.2.0
+ *
+ * @see WP_HTML_Tag_Processor
+ */
+class WP_HTML_Span {
+	/**
+	 * Byte offset into document where span begins.
+	 *
+	 * @since 6.2.0
+	 * @var int
+	 */
+	public $start;
+
+	/**
+	 * Byte offset into document where span ends.
+	 *
+	 * @since 6.2.0
+	 * @var int
+	 */
+	public $end;
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 6.2.0
+	 *
+	 * @param int $start Byte offset into document where replacement span begins.
+	 * @param int $end   Byte offset into document where replacement span ends.
+	 */
+	public function __construct( $start, $end ) {
+		$this->start = $start;
+		$this->end   = $end;
+	}
+}
+
+endif;