feat(rehype): add onFallback callback option

[dan-lee]

• <- Keep this line and put an x between the brackts.

Description

When using fallbackLanguage there's no way to know when a fallback actually happens, making it hard to detect typos in language identifiers or missing grammars.

Adds an onFallback callback to rehype options:

onFallback?: (context: { requestedLanguage: string; fallbackLanguage: string }) => void

So it can be used liked this:

rehypeShikiFromHighlighter(highlighter, {
  fallbackLanguage: "text",
  onFallback({ requestedLanguage, fallbackLanguage }) {
    console.warn(`Language "${requestedLanguage}" not found. Falling back to "${fallbackLanguage}"`);
  },
});

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[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.26%. Comparing base (2b33c0c) to head (50d997d).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/rehype/src/core.ts	54.54%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1250      +/-   ##
==========================================
- Coverage   95.26%   95.26%   -0.01%     
==========================================
  Files          92       92              
  Lines        8032     8040       +8     
  Branches     1714     1716       +2     
==========================================
+ Hits         7652     7659       +7     
- Misses        374      375       +1     
  Partials        6        6              

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[dan-lee]

I don't understand the issues from codecov.
I had to change these lines because the linter complained