Add kernel_javascript support to write kernel.js during kernelspec install

[blink1073]

References

Replaces and closes #194.

Description

Kernels can now set a kernel_javascript class attribute. When python -m my_kernel install runs, MetaKernel writes that string to kernel.js alongside kernel.json in the kernelspec directory, enabling CodeMirror syntax highlighting and other notebook-editor customisations without a separate packaging step.

Changes

• metakernel/_metakernel.py: instantiate the kernel class once in KernelInstallerApp.start(); if the instance has a non-empty kernel_javascript attribute, write it to kernel.js in the temporary kernelspec directory before calling jupyter kernelspec install.
• tests/test_metakernel_app.py: four new tests covering the happy path (file written, correct content), whitespace-only string (skipped), and absent attribute (skipped). Fixed three existing tests to set kernel_javascript = "" on their mock instances so the new branch does not interfere.
• docs/new_kernel.md: added a "Syntax highlighting (kernel.js)" section documenting the new attribute with a CodeMirror defineSimpleMode example.

Backwards-incompatible changes

None

Testing

New and existing tests in tests/test_metakernel_app.py all pass (just test tests/test_metakernel_app.py).

AI usage

• Some or all of the content of this PR was generated by AI.
• The human author has carefully reviewed this PR and run this code.
• AI tools and models used: Claude Sonnet 4.6 (Claude Code)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.84%. Comparing base (f9f5e94) to head (e0eb166).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
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+ Coverage   90.08%   90.84%   +0.75%     
==========================================
  Files          51       51              
  Lines        2944     2948       +4     
  Branches      411      412       +1     
==========================================
+ Hits         2652     2678      +26     
+ Misses        214      190      -24     
- Partials       78       80       +2     

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