feat: add `isISO6391` decorator for language codes

[RobinvanderVliet]

Description

Add isISO6391 validation decorator for ISO 639-1 language codes.

Checklist

• [x] the pull request title describes what this PR does (not a vague title like Update index.md)
• [x] the pull request targets the default branch of the repository (develop)
• [x] the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• [x] tests are added for the changes I made (if any source code was modified)
• [x] documentation added or updated
• [x] I have run the project locally and verified that there are no errors

Fixes

fixes #2045

[braaar]

Please fix the test so that we can proceed with this

[RobinvanderVliet]

@braaar I have added the new decorator to the exports. Now the tests run successfully on my machine.