fix(isLocale): match private-use and grandfathered tags case-insensitively - #2905

isLocale('X-WHATEVER'), isLocale('de-CH-X-phonebk'), and isLocale('I-AMI') currently return false, although their lowercase equivalents pass. The assembled regex uses case-flexible character classes for most subtags but matches the literal private-use prefix and grandfathered tags case-sensitively.

Compile the language-tag regex with the case-insensitive flag. Add regression coverage for mixed-case private-use prefixes, grandfathered tags, and malformed controls. This preserves the existing accepted syntax and underscore compatibility.

RFC 5646 §2.1.1 requires case-insensitive treatment of language tags and explicitly gives I-AMI as equivalent to i-ami: https://www.rfc-editor.org/rfc/rfc5646.html#section-2.1.1

Validation: the regression fails against the original source. npm test passes all builds, lint and 324 tests with 100% statement/function/line coverage; branch coverage remains 96.59%. Generated Node/browser/minified-browser checks also pass. Node 24.19.0 on Linux; the CI version matrix has not been run locally.

Prepared with OpenAI Codex assistance.

Checklist

• PR contains only related changes
• README unchanged: existing contract is restored
• Regression tests added
• Primary reference provided