Reimplements HtmlParser using Symfony DomCrawler

[JohnathonKoster]

This PR removes the RegEx-based parser and replaces it with PHP's HTML parser. Additionally, this PR utilizes Symfony's DomCrawler component to target the HTML nodes that should have their CSS class properties updated. Symfony's CssSelector component is also used to support more complex CSS-like selectors.

Backwards Compatibility

This PR maintains the existing configuration format, and converts them to their appropriate CSS style selectors. All existing test behavior has been preserved (with minor test changes to reflect the differences in how the emitted HTML attributes are ordered in the output).

An example of HTML attributes being emitted in a different order is:

<a class="link" href="#">Link</a>

now emits:

<a href="#" class="link">Link</a>

Improved Nested Element Support

This PR resolves #16 , and allows for deeply nested class configurations to be specified:

$config = [
    'default'  => [
        'a' => 'root-link',
        'ul' => 'parent',
        'ul li' => 'nested',
        'ul li a' => 'first-nested-links',
        'ul li ul' => 'nested-parent',
        'ul li ul li' => 'nested-item',
    ],
];

The above configuration could be used to produce output similar to the following example:

<a class="root-link">Root link</a>
<ul class="parent">
   <li class="nested"><a class="first-nested-links">I am nested 1</a></li>
   <li class="nested"><a class="first-nested-links">I am nested 2</a></li>
   <li class="nested"><a class="first-nested-links">I am nested 3</a></li>
   <li class="nested">
       <ul class="nested-parent">
           <li class="nested-item"><a class="root-link">I am nested 2.1</a></li>
           <li class="nested-item"><a class="root-link">I am nested 2.2</a></li>
           <li class="nested-item"><a class="root-link">I am nested 2.3</a></li>
       </ul>
   </li>
</ul>

By utilizing more complicated CSS-style selectors, developers may now be more specific in how they apply their CSS classes:

$config = [
    'default'  => [
        'a' => 'root-link',
        'ul' => 'parent',
        'ul li' => 'nested',
        'ul li:nth-child(2n+2)' => 'nested nested-even',
        'ul li a' => 'first-nested-links',
        'ul li ul' => 'nested-parent',
        'ul li ul li' => 'nested-item',
        'ul li ul li a' => 'deeply-nested-links',
    ],
];

Would produce the following output:

<a class="root-link">Root link</a>
<ul class="parent">
   <li class="nested"><a class="first-nested-links">I am nested 1</a></li>
   <li class="nested nested-even"><a class="first-nested-links">I am nested 2</a></li>
   <li class="nested"><a class="first-nested-links">I am nested 3</a></li>
   <li class="nested nested-even">
       <ul class="nested-parent">
           <li class="nested-item"><a class="deeply-nested-links">I am nested 2.1</a></li>
           <li class="nested-item"><a class="deeply-nested-links">I am nested 2.2</a></li>
           <li class="nested-item"><a class="deeply-nested-links">I am nested 2.3</a></li>
       </ul>
   </li>
</ul>

Improvements to CSS Framework Build Tooling Workflows

Additionally, this PR offers a solution for #20. While this PR does not directly solve the issue, it provides a new Artisan command that can be used to produce a JavaScript configuration file containing all of the Classify class names:

php artisan classify:export

This command will create a new classify.config.js file at the root of the project, with contents similar to:

module.exports.classes = [
    "mt-8",
    "first:mt-0",
    "text-xs",
    "uppercase",
];

This new configuration file can then be used to configure the CSS build process. For example, a Tailwind CSS version 3 configuration file could be updated to the following:

const classify = require('./classify.config');

module.exports = {
  content: [
    './pages/**/*.{html,js}'
    './components/**/*.{html,js}',
  ],
  safelist: [
    ...classify.classes,
    'bg-red-500',
    'text-3xl',
    'lg:text-4xl',
  ]
  // ...
}