Add support for nonExplicitSupportedLngs

[chrilis]

This PR adds support for nonExplicitSupportedLngs and is related to #1412. This features adds the possibility to support multiple variants of a language (de-AT, de-CH and de-DE) with a single JSON file (e.g. public/locales/de/common.js).

During the work on this PR I noticed that the the default value in fallbackLng should always be loaded. At least this is how I interprete this testcase. Although this is related to #1927, the fix is part of this PR, because nonExplicitSupportedLngs depends on the same logic. Regarding the test-case above: I added en-US to supported locales and removed en from the initialI18nStore, because I think this should be the expected behavior. Is this correct @adrai?

Currently the combination fallbackLng with a function and nonExplicitSupportedLngs: true is not supported. Mainly because I didn't know how this could be implemented 😅 But I guess that the use cases for this combination should be very rare.

I'm looking forward to your feedback @adrai, thank you!

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• run tests npm run test
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[adrai]

lgtm

[adrai]

This looks really good to me.
Do you think this could be released with a minor version, or should we go with a major?

[chrilis]

I'm not sure. The changes could lead to more languages being loaded, which would increase the page size. I tend to think of that as a breaking change - what do you think?

[adrai]

just released v12.0.0... thank you very much for your great contribution @chrilis.