Implement BOM stripping and ignoreBOM option in TextDecoder

[apoorvdarshan]

Summary

• Strip BOM bytes by default during decoding (UTF-8, UTF-16LE, UTF-16BE) per the WHATWG Encoding spec
• Support the ignoreBOM constructor option to opt out of stripping
• Add read-only ignoreBOM getter property

Closes #4613

Test plan

• Added tests for default BOM stripping (UTF-8, UTF-16LE, UTF-16BE)
• Added tests for ignoreBOM: true preserving BOM for all three encodings
• Added tests for ignoreBOM: false stripping BOM (same as default)
• Added test for ignoreBOM getter returning correct values
• All 50 boa_runtime tests pass with zero warnings

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,472	49,471	-1
Ignored	2,249	2,249	0
Failed	1,141	1,142	+1
Panics	0	0	0
Conformance	93.59%	93.59%	-0.00%

Broken tests (1):

test/intl402/NumberFormat/prototype/resolvedOptions/basic.js (previously Passed)

[codecov]

Codecov Report

❌ Patch coverage is 90.78947% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.58%. Comparing base (6ddc2b4) to head (177202b).
⚠️ Report is 660 commits behind head on main.

Files with missing lines	Patch %	Lines
core/runtime/src/text/mod.rs	72.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4631      +/-   ##
==========================================
+ Coverage   47.24%   56.58%   +9.33%     
==========================================
  Files         476      548      +72     
  Lines       46892    60064   +13172     
==========================================
+ Hits        22154    33987   +11833     
- Misses      24738    26077    +1339     

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[jedel1043]

Thank you for the contribution! It looks very good, I just have a suggestion to simplify the new tests.

[apoorvdarshan]

Thanks for the review! Refactored the 9 individual BOM tests into 3 parameterized #[test_case] functions, following the roundtrip pattern. Also fixed the rustfmt issue from the earlier push.

[jedel1043]

Looks great!