feat(yaml): introduce YamlSyntaxError with structured position info

[tomas-zijdemans]

Introducing YamlSyntaxError mirroring XmlSyntaxError from @std/xml:

• YamlSyntaxError extends SyntaxError with line, column, offset, snippet
• parse() and parseAll() now throw it (including the multi-document case, with position pointing at the second ---)
• onWarning callback type tightened to YamlSyntaxError
• error.message format unchanged. Fully backward-compatible

[codecov]

Codecov Report

❌ Patch coverage is 97.56098% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.85%. Comparing base (f017697) to head (e0abc68).

Files with missing lines	Patch %	Lines
yaml/_loader_state.ts	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7156      +/-   ##
==========================================
+ Coverage   94.84%   94.85%   +0.01%     
==========================================
  Files         617      618       +1     
  Lines       51422    51690     +268     
  Branches     9300     9353      +53     
==========================================
+ Hits        48769    49032     +263     
- Misses       2112     2113       +1     
- Partials      541      545       +4     

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