Add line numbers and line highlighting for code blocks

[dereuromark]

Summary

• Add VitePress-inspired syntax for code block line numbers and line highlighting
• Shorthand # syntax for line numbers (cleaner than :line-numbers)
• Support for line number offset with #=N
• Support for line highlighting with {1,3-5,7} ranges
• All features can be combined: ```php #=5 {2,4}

Syntax Examples

Syntax	Result
```php	Plain code block
```php #	With line numbers
```php #=5	Line numbers starting at 5
```php {3,5-7}	Highlight lines, no numbers
```php # {3,5-7}	Line numbers + highlighting
```php #=5 {3,5-7}	Numbers from 5 + highlighting

HTML Output

When line numbers or highlighting are enabled, the renderer:

• Adds line-numbers class to <pre> when line numbers enabled
• Adds data-start="N" attribute when start offset != 1
• Adds has-highlighted-lines class when highlighting used
• Wraps each line in <span class="line" data-line="N">
• Adds highlighted class to highlighted line spans

Example output:

<pre class="line-numbers has-highlighted-lines" data-start="5"><code class="language-php"><span class="line" data-line="5">$foo = 1;</span>
<span class="line highlighted" data-line="6">$bar = 2;</span>
<span class="line" data-line="7">$baz = 3;</span>
</code></pre>

CSS can then style line numbers via counters and highlighted lines with background colors.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.72%. Comparing base (9b26950) to head (0bdc767).

Files with missing lines	Patch %	Lines
src/Node/Block/CodeBlock.php	50.00%	6 Missing ⚠️
src/Renderer/HtmlRenderer.php	85.71%	6 Missing ⚠️
src/Parser/Block/FencedBlockParser.php	97.67%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #82      +/-   ##
============================================
- Coverage     93.85%   93.72%   -0.13%     
- Complexity     2115     2149      +34     
============================================
  Files            77       77              
  Lines          5675     5773      +98     
============================================
+ Hits           5326     5411      +85     
- Misses          349      362      +13     

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[dereuromark]

Closing this PR as the functionality is better handled by a dedicated syntax highlighter.

After further consideration, line numbers and line highlighting should be handled by Phiki/Torchlight Engine rather than by djot-php:

1. Separation of concerns - djot-php should focus on parsing Djot markup, not code rendering
2. Feature richness - Torchlight provides much more: diff markers, focus mode, collapse, annotations via [tl! ...] comments
3. Theme support - Torchlight uses VS Code themes for consistent styling
4. Better architecture - Extensions can hook into djot-php's render.code_block event to use any highlighter

The wp-djot plugin now uses Torchlight Engine via a custom extension:

• torchlight/engine added as composer dependency
• TorchlightExtension hooks into render.code_block event
• All highlighting/line numbers/annotations handled by Torchlight

See: https://github.com/torchlight-api/engine