pr: make -n/--number-lines argument optional to match GNU

[AlejandroCoronadoN]

Problem

GNU pr treats the -n/--number-lines argument as optional. uutils required
a value, so a bare flag failed:

$ printf 'a\nb\n' | pr -t -n
error: a value is required for '--number-lines <[char][width]>' but none was supplied

Fix

Add num_args(0..=1) to the argument. When no value is given, the existing
.or_else(NumberingMode::default()) fallback already supplies the default
(width 5, tab separator), so no further change is needed.

Verification

Compared against GNU coreutils pr over -n, --number-lines, -n5, -n:3,
-nx3, and -n FILE; all match byte for byte.

$ printf 'a\nb\n' | pr -t -n
    1	a
    2	b

--number-lines= (explicit empty value) is still rejected. Added a regression
test; the full test_pr suite (239 tests) passes and cargo fmt / cargo clippy are clean.

[codspeed-hq]

Merging this PR will regress 1 benchmark

⚠️ 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 improved benchmark
❌ 1 regressed benchmark
✅ 345 untouched benchmarks
🆕 10 new benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	numfmt_large_numbers_si[10000]	94.7 ms	98.1 ms	-3.4%
⚡	Simulation	complex_relative_date	330.2 µs	318.6 µs	+3.65%
🆕	Memory	ptx_input_references_short_lines[100000]	N/A	222.2 MB	N/A
🆕	Memory	ptx_long_lines[100]	N/A	29.5 MB	N/A
🆕	Memory	ptx_short_lines[100000]	N/A	240.1 MB	N/A
🆕	Memory	ptx_input_references_long_lines[1000]	N/A	29.1 MB	N/A
🆕	Memory	ptx_tex[10000]	N/A	20.6 MB	N/A
🆕	Simulation	ptx_input_references_short_lines[100000]	N/A	16.1 s	N/A
🆕	Simulation	ptx_long_lines[100]	N/A	1.7 s	N/A
🆕	Simulation	ptx_short_lines[100000]	N/A	14 s	N/A
🆕	Simulation	ptx_input_references_long_lines[1000]	N/A	2 s	N/A
🆕	Simulation	ptx_tex[10000]	N/A	2.9 s	N/A

Tip

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

---

_{Comparing AlejandroCoronadoN:fix-pr-number-lines-optional (6a8c3d3) with main (66cf72b)²}

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. ↩

2. No successful run was found on main (584dc43) during the generation of this report, so 66cf72b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩