fix: Improve CodeGroupExtension with better language parsing and attribute preservation

[dereuromark]

Summary

This PR improves the CodeGroupExtension with changes aligned with djot-php's upstream implementation.

Bug Fixes

• Language regex now supports special characters (c++, c#, text/html, etc.)
  • Old regex: /[A-Za-z0-9_-]+/ - only alphanumeric, underscore, dash
  • New regex: /[^\s\[]+/ - any non-whitespace characters

New Features

• Attribute preservation: custom id, classes, and data-* attributes from the div are now preserved in the rendered output
• Multiple custom classes are merged with glaze-code-group class

Example

{#my-code .custom-theme data-section="intro"}
::: code-group
``` php
$config = [];

:::

Now renders:
```html
<div class="glaze-code-group custom-theme" id="my-code" data-section="intro" role="tablist">
  ...
</div>

Dependencies

Requires: php-collective/djot ^0.1.21 for attribute preservation to work (contains fix for class merging in fenced divs - php-collective/djot-php#137)

Related

• Aligned with upstream: feat: Add CodeGroupExtension for tabbed code block interfaces php-collective/djot-php#132
• Depends on fix: fix: Merge div fence class with attribute block classes php-collective/djot-php#137