Potential expansion to be less MusicBrainz-centric

Currently, most of the extensions depend on being attached to a particular MusicBrainz entity, and don't offer many of their own fields on the root Query.

I'm considering rebranding this project to something like "MusiQL", which would be very similar, but not MusicBrainz-centric. Most of the existing queries would work exactly the same, but be under a `musicBrainz` field on the root query.