Locale does not respect numeric singleton extensions. Currently `LanguageTag` which handles singleton extension parsing only allows `alpha` characters when it should allow `alphaNumeric`.
This PR corrects this and adds tests for `forLanguageTag` as well as the builder. --------- - [x] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). ------------- Commit messages: - init Changes: https://git.openjdk.org/jdk/pull/31644/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=31644&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8387185 Stats: 19 lines in 2 files changed: 15 ins; 0 del; 4 mod Patch: https://git.openjdk.org/jdk/pull/31644.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/31644/head:pull/31644 PR: https://git.openjdk.org/jdk/pull/31644
