Add support for line numbers in Markdown code fences

[coreyasmith]

Older versions of Ghost (2 and earlier?) used to support line numbers in Markdown code fences by adding line-numbers after the language:

```ruby line-numbers
foo = "bar"
```

This would be rendered as:

<pre>
  <code class="line-numbers language-ruby">
  foo = "bar"
  </code>
</pre>

The Prism Line Numbers plugin would then render line numbers for that code block.

This pull request adds in a markdown-it plugin that I implemented called markdown-it-fence-line-numbers to restore this functionality to Ghost's Markdown cards.

[coderabbitai[bot]]

Walkthrough

The PR adds a new function fenceLineNumbers(md) that augments fenced code rendering by detecting the line-numbers token in a fence's info string and appending the line-numbers class when present, otherwise delegating to the default fence renderer. selectRenderer is updated to apply this plugin for the latest Ghost renderer path (4.x+). Tests were added to assert that fenced blocks include class="line-numbers language-..." when requested and remain class="language-..." otherwise; behavior for Ghost <4.x is unchanged.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title succinctly and accurately describes the primary change introduced by the pull request, namely adding support for line numbers in Markdown code fences, and is fully aligned with the code modifications and objectives.
Description Check	✅ Passed	The description clearly explains the previous behavior, the motivation for restoring line-number support, and how the new markdown-it plugin implementation achieves this, directly reflecting the changes in the PR.
Docstring Coverage	✅ Passed	No functions found in the changes. Docstring coverage check skipped.

✨ Finishing touches

• [ ] 📝 Generate Docstrings

🧪 Generate unit tests

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment

[!TIP]

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[codecov-commenter]

Codecov Report

:x: Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 90.88%. Comparing base (8c3fc1c) to head (733f4ef). :warning: Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rkdown-html-renderer/lib/markdown-html-renderer.js	87.09%	4 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1381   +/-   ##
=======================================
  Coverage   90.87%   90.88%           
=======================================
  Files         188      188           
  Lines       17887    17917   +30     
  Branches     2000     2006    +6     
=======================================
+ Hits        16254    16283   +29     
- Misses       1622     1623    +1     
  Partials       11       11           

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