[localize] Reversish locale

It would be helpful to be able to do an e2e test of a lit localize setup before you have any translations. We can add a fake locale called "reversish" that automatically generates translations by reversing all of the strings (or some other trivial transform could also work).

No file or test is named. Start by locating the lit localize setup and existing end-to-end tests; add the proposed “reversish” fake locale that reverses strings, then verify that an e2e setup works without translations.

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