new validation rule: valid bcp 47 locale/language tag

[samuelstroschein]

Context

Users are running into bugs because they use invalid BCP47 tags opral/inlang-fink#42 (comment).

We previously validated the locales/language tag as part of parsing the settings file. That was reversed because some users might not or want to use BCP47.

Proposal

Introduce a BCP47 language tag rule that validates the locale/language tag in the settings, and messages of an inlang.

• prime validation rule use case. different formats can be valid, let users decide what rule they want to enforce.

[NiklasBuchfink]

This has happened here before: https://discord.com/channels/897438559458430986/1260961041983082496/