Split some Language methods to LanguageNameUtils
These are static methods that have to do with processing language names
and codes. I didn't include fallback behavior, because that would mean a
circular dependency with LocalisationCache.
In the new class, I renamed AS_AUTONYMS to AUTONYMS, and added a class
constant DEFINED for 'mw' to match the existing SUPPORTED and ALL. I
also renamed fetchLanguageName(s) to getLanguageName(s).
There is 100% test coverage for the code in the new class.