We still need to force the v3 format internally because i18next-scanner
doesn't support the new format and v4 drops support for Safari<13.

By setting compatibilityJSON things should continue to work as before.

Instead of using the namespace which is hardcoded we use the tag name that was
used to register the element (or the "real" tag name in case of a scoped element).

This way if AppShell is regsitered as "dbp-some-app" it can still be overriden by
specifying "dbp-some-app" in the overrides object.

We use i18n.languages in the app-shell to see which languages are available
but this only contains a list of languages it will search in (means, currently
set language, default one and all fallbacks).

We can just set all languages we know as last fallback, so i18n.languages
always contains all languages.

A bit hacky...

Create a new instance with createInstance() and call setOverrides()
with override data to override translations.

See the tests for some examples.

Uses a separate namespace that gets used over the default one, so we can override,
but also remove the overrides again when needed.

This has the problem that all clones have the same store, so this overrides
the translations for all clones and not just the one providing the overrides.