feat(rehype): Support fallbackLanguage on lazy mode

[fuma-nama]

Description

Support fallbackLanguage when lazy is set to true, you can consider this as a successor of #821.

Since highlighter.loadLanguage throws error on languages that's not included in the bundle, we can use it to detect whether fallback is needed.

Linked Issues

N/A

Additional context

Unit test added

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[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.23%. Comparing base (c55865f) to head (93a5cf0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/rehype/src/core.ts	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #912      +/-   ##
==========================================
- Coverage   94.27%   94.23%   -0.04%     
==========================================
  Files          91       91              
  Lines        7471     7481      +10     
  Branches     1535     1539       +4     
==========================================
+ Hits         7043     7050       +7     
- Misses        419      422       +3     
  Partials        9        9              

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