fix(lint/useNamingConvention): accept unicase characters

[Conaclos]

Summary

Close #1786

unicase letters have a single case: they are neither uppercase nor lowercase.
Previously, Biome reported names in unicase as invalid.
It now accepts a name in unicase everywhere.

The following code is now accepted:

const 안녕하세요 = { 안녕하세요: 0 };

We still reject a name that mixes unicase characters with lowercase and uppercase characters:
The following names are rejected:

const A안녕하세요 = { a안녕하세요: 0 };

I also removed some dead code in Case::convert and improved the documentation of Case::is_compatible.

Test Plan

• New tests added.

[netlify]

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Lighthouse	1 paths audited Performance: 100 (🟢 up 4 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 93 (no change from production) PWA: - View the detailed breakdown and full score reports

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[codspeed-hq]

CodSpeed Performance Report

Merging #1806 will degrade performances by 7.82%

_{Comparing conaclos/lint/useNamingConvention/1786 (e4296bd) with main (dca6a7a)}

Summary

❌ 1 regressions
✅ 92 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	conaclos/lint/useNamingConvention/1786	Change
❌	eucjp.json[uncached]	5 ms	5.5 ms	-7.82%