fix TextDecoder label parsing

[Monti-27]

Fixes #5135.

This makes TextDecoder normalize supported encoding labels before matching them.

It now accepts supported labels with ASCII case differences, surrounding ASCII whitespace, and the supported aliases for the UTF-8 and UTF-16 encodings Boa currently implements.

Tests:

• cargo test -p boa_runtime text --lib
• cargo test -p boa_runtime --lib
• cargo clippy -p boa_runtime --all-features --all-targets -- -D warnings

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,079	50,079	0
Ignored	2,072	2,072	0
Failed	812	812	0
Panics	0	0	0
Conformance	94.55%	94.55%	0.00%

Tested main commit: dd1efc2cef0868102f9e29cc9d05960c8283046b
Tested PR commit: 6bb998b1a73b1420bea8282ff770876ee001e91f
Compare commits: dd1efc2...6bb998b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.57%. Comparing base (6ddc2b4) to head (495d948).
⚠️ Report is 895 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5162       +/-   ##
===========================================
+ Coverage   47.24%   59.57%   +12.33%     
===========================================
  Files         476      580      +104     
  Lines       46892    63257    +16365     
===========================================
+ Hits        22154    37688    +15534     
- Misses      24738    25569      +831     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[jedel1043]

Looks good! I just have a couple of nitpicks

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,079	50,079	0
Ignored	2,072	2,072	0
Failed	812	812	0
Panics	0	0	0
Conformance	94.55%	94.55%	0.00%

Tested main commit: c9aaad16b19aa2cadf72838fe8acc1043013023c
Tested PR commit: 495d948fd3881052eb332f6bb865ea1fbc31d5bf
Compare commits: c9aaad1...495d948

[jedel1043]

Looks great!

[Monti-27]

Looks great!

thanks mentor !!