feat(yaml): stabilize quoteStyle option for stringify()

[tomas-zijdemans]

Promotes the quoteStyle option from unstable_stringify.ts into the stable StringifyOptions. No behavior change: DumperState already handled the option, so this only moves the type, its doc comment, and the test.

quoteStyle has been unstable since May 2025 (#6666). No open issues against it, no changes since it landed. It has baked long enough.

extraTypes stays unstable on purpose: its shape may still change depending on how #7020 (explicit tag support) resolves.

Tested by switching the existing quoteStyle test from the unstable import to the stable one. All 45 tests in yaml/stringify_test.ts pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (a3e93a0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7288      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      617              
  Lines       51637    51637              
  Branches     9359     9359              
==========================================
- Hits        49075    49071       -4     
- Misses       2021     2023       +2     
- Partials      541      543       +2     

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