Add a `bin/i18n validate` workflow to check for errors in translation files
Summary:
Part of T16378. Depends on D26559 (for the `getAllTranslations` method)
Someday I may turn this into a unit test that ensures there are no validation errors. But not yet - the patches to fix all of the validation errors are still in review or yet to be written.
Test Plan: Run the script via `./bin/i18n validate`, see various errors. Apply various in-review patches (D26548, D26558, D26546, D26574) and see fewer errors with each patch applied. With all of those patches applied (and the downstream translations extension not installed since it doesn't follow these conventions yet) see no validation errors at all.
Reviewers: O1 Blessed Committers, aklapper
Reviewed By: O1 Blessed Committers, aklapper
Subscribers: aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26572