Fix encoding canonical names to match WHATWG spec

[woustachemax]

Changes both implementation and tests to use canonical encoding names:

• 'ibm-866' - 'ibm866'
• 'shift-jis' - 'shift_jis'
  Per WHATWG Encoding Standard, TextDecoder.encoding should return the canonical encoding name, not the label used to construct it.

Fixes #5663

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[woustachemax]

I have read the CLA Document and I hereby sign the CLA

[codspeed-hq]

Merging this PR will degrade performance by 18.79%

Summary

❌ 1 regressed benchmark
✅ 128 untouched benchmarks
⏩ 49 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	jsonResponse[Response]	34.6 µs	42.7 µs	-18.79%

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_{Comparing woustachemax:fix-encoding-canonical-names (2977ef0) with main (6bacc75)}

Footnotes

1. 49 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩