Internationalization: custom script for iOS with fbt

[albullington]

If we end up going with fbt as our internationalization library, we'll need a custom script that can take translations and turn them into iOS localized strings.

Currently, there's a script that does this for Android strings (step 4 in the React Native demo app). The iOS version should work similarly, but we'll need to figure out how to include plural strings / strings with variables in the Localizable.strings format. (I made an issue for this here, but no response from the fbt team).

We should explore this to help us make a decision on which internationalization library to use.