feat(app): Add 'getAppLanguageCode' and 'getAppLanguageTag'

[philet]

Solves #1693

[jcesarmobile]

I've addressed the changes requested by Mike and added two buttons on the test app for testing
ionic-team/capacitor-testapp#628

But I've noticed that getAppLanguageTag() is also returning a 2 language code on iOS instead of the documented "IETF BCP 47 language tag".
Bundle.main.preferredLocalizations.first will return a matching code on CFBundleLocalizations if there is one, which is usually a 2 letter code, or if there isn't one that matches, then it will return the CFBundleDevelopmentRegion, which is a 2 or 3 letter ISO code. So I don't think the getAppLanguageTag method is needed, and also, I don't think the getAppLanguageCode should return a 2 letter code as it could also be a 3 letter ISO code.
I think it should just be getAppLanguage() and return the whole value of Bundle.main.preferredLocalizations.first

[IT-MikeS]

Please see jcesarmobile's comment above.

[ThatOneCalculator]

Any update on this?