numfmt: cap IEC precision at 3 decimals

[pocopepe]

Fixes #11926

Added effective_precision = precision.min(3) for iec units in consider_suffix to match gnu's rounding behaviour. Tests added for both the IEC cap and that SI is unaffected.
Verified against GNU 9.4.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Congrats! The gnu test tests/cp/link-heap is now passing!
Congrats! The gnu test tests/cut/bounded-memory is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 240 untouched benchmarks
⏩ 115 skipped benchmarks¹

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_{Comparing pocopepe:11926 (9c0bf0e) with main (cdcbee3)}

Footnotes

1. 115 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. ↩