cut: support multibyte characters in non-UTF-8 locales

[sylvestre]

Add locale-aware character handling so -c counts characters, -b -n keeps multibyte characters whole, and -d accepts a single multibyte delimiter.

Should make test tests/cut/mb-non-utf8.sh pass

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/cut/mb-non-utf8 is no longer failing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[codspeed-hq]

Merging this PR will degrade performance by 87.13%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 322 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cut_characters	7.4 ms	57.6 ms	-87.13%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing sylvestre:mb-non-utf8 (350f371) with main (43ef8d0)}

Footnotes

1. 46 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. ↩