feat(front-matter/unstable): add test to each subpackage, including JSON, TOML, and YAML

[uynilo9]

Let people use the test function more easily when they want to check a specific format of frontmatter.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 62.96296% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.36%. Comparing base (260f5b8) to head (848bea9).

Files with missing lines	Patch %	Lines
front_matter/unstable_json.ts	54.54%	5 Missing ⚠️
front_matter/unstable_toml.ts	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7059      +/-   ##
==========================================
- Coverage   94.38%   94.36%   -0.02%     
==========================================
  Files         628      630       +2     
  Lines       50175    50202      +27     
  Branches     8840     8840              
==========================================
+ Hits        47357    47374      +17     
- Misses       2251     2261      +10     
  Partials      567      567              

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