feat: implement ECMA-426 source map v3 decoder

[countradooku]

Summary

• add new boa_sourcemap crate implementing ECMA-426 source map decoding
• implement high-performance base64 VLQ + mappings decoder
• support regular maps, index maps (sections), source resolution, and ignore list handling
• add JavaScript sourceMappingURL extraction utility
• add tests for VLQ, mappings decode, index map offsets, ignore list fallback, and URL extraction

Validation

• cargo test -p boa_sourcemap

Closes #4562

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,504	49,504	0
Ignored	2,262	2,262	0
Failed	1,096	1,096	0
Panics	0	0	0
Conformance	93.65%	93.65%	0.00%

[codecov]

Codecov Report

❌ Patch coverage is 82.28438% with 76 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.35%. Comparing base (6ddc2b4) to head (0ce8663).
⚠️ Report is 688 commits behind head on main.

Files with missing lines	Patch %	Lines
core/sourcemap/src/lib.rs	82.28%	76 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4736       +/-   ##
===========================================
+ Coverage   47.24%   57.35%   +10.10%     
===========================================
  Files         476      550       +74     
  Lines       46892    60829    +13937     
===========================================
+ Hits        22154    34887    +12733     
- Misses      24738    25942     +1204     

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[jedel1043]

Marking as blocked since we didn't properly triage #4562