This adds a dedicated `AvailableLanguages` struct in preparation for a
more cabable language matching based on a given language config. No
functional changes, just shuffling some code around for this round.

Also made the LanguageMatcher non-cloneable in favor of wrapping it in
an Arc, since cloning is rather expensive for this and having a
reference is sufficient in all cases right now.

Release Notes:

- N/A

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Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>