tac: support non-UTF-8 separator

[victor-prokhorov]

fixes #9502

open question: GNU tac also accepts non-UTF-8 separators in regex mode. right now, in this pr i return an error. should i handle this in the same PR, or i should do it in another?

[ChrisDryden]

The ultimate goal it to get the non utf-8 stuff to be fully matching how gnu handles it, can you try to handle it in the same pr?

[codspeed-hq]

Merging this PR will not alter performance

✅ 288 untouched benchmarks
⏩ 38 skipped benchmarks¹

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_{Comparing victor-prokhorov:tac-non-utf8-separator (9bfe761) with main (def3956)}

Footnotes

1. 38 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. ↩

[ChrisDryden]

By fixing this it should also fix the tac-locale test so you can use that to guide you on whether the fix is working properly