perf(yaml): use direct assignment for mapping keys

[tomas-zijdemans]

• storeMappingPair and mergeMappings now use direct property assignment instead of Object.defineProperty for ordinary keys. The descriptors defineProperty was writing ({writable, enumerable, configurable} all true) are equivalent to plain assignment, and defineProperty is significantly slower in V8.
• __proto__ still goes through Object.defineProperty to keep the prototype-chain protection in runtimes that enable the Object.prototype.__proto__ setter.

Based on a wide array of yaml files, I see a consistent 1.24x to 1.43x improvement in mapping-heavy YAML, with no regressions in other payloads

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (cd03740) to head (77d2a70).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7126   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         634      634           
  Lines       51799    51807    +8     
  Branches     9329     9334    +5     
=======================================
+ Hits        49009    49017    +8     
  Misses       2216     2216           
  Partials      574      574           

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