Provide a parameter that can support multi-arb folder.

[dodatw]

I'm facing a classic problem that packages don't support international translations.
Please refer to: localizely/flutter-intl-intellij#22

I moved some common functionality into some packages, but it can't have its own translations.
Only app projects have translations because
"The application cannot load multiple delegate files because a CompositeMessageLookup has already been instantiated"

I found that if I copy the translation strings from the package to the main project it works. (because these translations are added when the application CompositeMessageLookup is initialized)

But I don't want copy manually everytime, I have an idea, is it possible to support multiple arb folders so that the main app can have all translation?

Something like:

flutter_intl:
  enabled: true
  other_arb_dirs: package/moduleA/l10n, package/moduleB/l10n