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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.38%. Comparing base (260f5b8) to head (bc74fa0).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7059   +/-   ##
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  Coverage   94.38%   94.38%           
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  Files         628      630    +2     
  Lines       50175    50190   +15     
  Branches     8840     8840           
=======================================
+ Hits        47357    47372   +15     
  Misses       2251     2251           
  Partials      567      567           

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[bartlomieju]

A few things to address:

1. unstable_json.ts and unstable_toml.ts duplicate extract() from the stable json.ts and toml.ts. These files should only contain the new test() function — either re-export extract from the stable module or just don't include it.
2. The JSON test cases use TOML syntax (name = 'deno') as the front matter body. Since test() only validates delimiters this passes, but it's confusing — please use actual JSON content.
3. Import style is inconsistent: json_test.ts uses the @std/front-matter/unstable-json specifier while toml_test.ts uses a relative ./unstable_toml.ts path. Please use one style consistently.

[uynilo9]

Thanks for reviewing. I didn't realise some absurd mistakes I made since I've been little busy these days.

2. The JSON test cases use TOML syntax (`name = 'deno'`) as the front matter body. Since `test()` only validates delimiters this passes, but it's confusing — please use actual JSON content.

About the second thing you mention, I use the example directly from test_test.ts. Is it also a misused test?