csplit: add benchmarks for line number and regex pattern splitting #10927

Performance Gate Passed

✅ 284 untouched benchmarks
🆕 4 new benchmarks
⏩ 38 skipped benchmarks¹

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
🆕	Memory	`csplit_regex_pattern`	N/A	385.6 KB	N/A
🆕	Memory	`csplit_line_number`	N/A	339.7 KB	N/A
🆕	Simulation	`csplit_regex_pattern`	N/A	35 ms	N/A
🆕	Simulation	`csplit_line_number`	N/A	28.6 ms	N/A

_{Comparing sylvestre:csplit-bench (402ca02) with main (eea061d)}

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. ↩