Parser: Parse <!-- more --> tag and <!-- noteaser -->

These tags are officially supported by WordPress and are an exception
with the concept of blocks. They contain structural data and yet they
are just HTML comments without any further indication.

The "more" tag supports an optional custom text string and if it is
immediately followed by the "no teaser" tag then that contains
additional information.

In this change we parse those tags and combine them into a single "more"
block with attributes of the custom text and whether or not the
`noTeaser` option has been selected.

blocks/api/post.pegjs

@@ -22,10 +22,25 @@ WP_Block_List
   = WP_Block*
 
 WP_Block
-  = WP_Block_Void
+  = WP_Tag_More
+  / WP_Block_Void
   / WP_Block_Balanced
   / WP_Block_Html
 
+WP_Tag_More
+  = "<!--" WS* "more" customText:(WS+ text:$((!(WS* "-->") .)+) { return text })? WS* "-->" noTeaser:(WS* WP_Tag_NoTeaser)?
+  { return {
+    blockName: 'wp:core/more',
+    attrs: {
+      customText: customText,
+      noTeaser: !! noTeaser
+    },
+    rawContent: ''
+  } }
+
+WP_Tag_NoTeaser
+  = "<!--" WS* "noteaser" WS* "-->"
+
 WP_Block_Void
   = "<!--" WS+ "wp:" blockName:WP_Block_Name WS+ attrs:(a:WP_Block_Attributes WS+ {
     /** <?php return $a; ?> **/

post-content.js

@@ -12,6 +12,9 @@ window._wpGutenbergPost = {
 			'<section className="cover-image wp-block-cover-image" style={ { backgroundImage: \'url("https://cldup.com/GCwahb3aOb.jpg");\' } }><h2>Of mountains & printing presses</h2></section>',
 			'<!-- /wp:core/cover-image -->',
 
+			'<!-- more Keep on reading! -->',
+			'<!-- noteaser -->',
+
 			'<!-- wp:core/text { "project": { "name": "gutenberg", "for": "WordPress" }, "isAwesome": true } -->',
 			'<p>The goal of this new editor is to make adding rich content to WordPress simple and enjoyable. This whole post is composed of <em>pieces of content</em>—somewhat similar to LEGO bricks—that you can move around and interact with. Move your cursor around and you\'ll notice the different blocks light up with outlines and arrows. Press the arrows to reposition blocks quickly, without fearing about losing things in the process of copying and pasting.</p>',
 			'<!-- /wp:core/text -->',