feat(i18n): add zh-TW (Traditional Chinese, Taiwan) locale

[PeterDaveHello]

This pull request adds support for Traditional Chinese (Taiwan) as a separate locale (zh-TW), improves locale detection logic for Chinese variants, and updates translation files and documentation accordingly.

The changes ensure users can explicitly select between Simplified Chinese (zh-CN), Traditional Chinese for Hong Kong (zh-HK), and Traditional Chinese for Taiwan (zh-TW), and that the correct translations are loaded based on their system or user-selected locale.

[pullfrog]

You've hit your limit · resets 5pm (UTC)

  ^{｜ Rerun failed job ➔ ｜ View workflow run ｜ via Pullfrog ｜ Using Claude Opus ｜ 𝕏}

[AprilNEA]

Thanks for adding zh-TW. I think this needs a small follow-up before merging: clippy is currently failing, and there are a couple of i18n edge cases/comments to fix inline.

[pullfrog]

✅ No new issues found.

Reviewed changes — first Pullfrog pass on this PR; the force-pushed commit folds in fixes for all three of AprilNEA's prior threads on i18n.rs.

• Rewrote match_supported's zh arm as a single-pass script/region scan — clears the Vec-allocation clippy lint and expresses resolution priority explicitly: Hans → zh-CN, hk/mo region → zh-HK, tw region or bare Hant → zh-TW, fallback → zh-CN.
• Documented the resolution order on match_supported — including the corner case that zh-Hans-HK stays Simplified (script wins) while zh-Hant-HK resolves to HK (region wins over generic Hant).
• Corrected the SUPPORTED doc comment — zh-TW is not in gpui-component's bundled ui.yml, so it's listed alongside ja / ru as locales where framework widget strings fall back to English.
• Expanded match_supported tests — added zh-Hans-HK, zh-Hant-TW, zh-TW, zh-Hant, and zh-Hant-HK cases covering all four match arms.
• Added zh-TW coverage to locale_file_resolves_keys — asserts Settings, Left Click, the interpolated Bind %{name}, and a BLURB fallback check so a missing/typoed zh-TW column would fail in CI.
• Renamed the picker labels for disambiguation — zh-HK → 繁體中文（香港）, zh-TW → 正體中文（臺灣）, using Taiwan's preferred 正體 term.
• Added zh-TW: translations across locales/app.yml — every previously translated key now has a Taiwan-flavor column.

  ^{｜ View workflow run ｜ Using Claude Opus ｜ 𝕏}