fold: add a benchmark

[sylvestre]

No description provided.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #8801 will not alter performance

_{Comparing sylvestre:bench-fold (7120f85) with main (b4f8eac)}

Summary

✅ 65 untouched
🆕 2 new
⏩ 73 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	fold_custom_width[50000]	N/A	60.8 ms	N/A
🆕	fold_many_lines[100000]	N/A	153.1 ms	N/A

Footnotes

1. 73 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. ↩