Unexpand: use byte count for multibyte characters for column width when using -a flag

[JaneIllario]

Salut, merci pour la review! 😄

In #9948 the unexpand utilities diverges from the GNU behavior with multi-byte characters. This fix replaces the UnicodeWidthChar with nbytes to maintain the same compatibility with the GNU unexpand. I also added an integration test based on the busybox test and the issue description to make sure that we don't regress to the same behavior.

[codspeed-hq]

CodSpeed Performance Report

Merging #9949 will improve performance by 6.85%

_{Comparing JaneIllario:unexpand-unicode (2ded1fe) with main (fd68328)¹}

Summary

⚡ 2 improvements
✅ 134 untouched
⏩ 15 skipped²

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	unexpand_many_lines[100000]	269.6 ms	252.3 ms	+6.85%
⚡	unexpand_large_file[10]	565.2 ms	529 ms	+6.85%

Footnotes

1. No successful run was found on main (c8c412c) during the generation of this report, so fd68328 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 15 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. ↩