mediawiki.language: Expose language fallback chain

The data can now be accessed via mw.language#getFallbackLanguages or
mw.language#getFallbackLanguageChain.

Bug: 69285
Change-Id: Id0c3267f01d5c4da0e4000a0fb318a9dfb538ef3
Merge "mediawiki.language: Expose language fallback chain"