ItemKey: Add ItemKey::MusicBrainzReleaseType

[matteron]

Hiya, needed this for reading album types (album vs ep vs single, etc).

https://picard-docs.musicbrainz.org/en/appendices/tag_mapping.html#id32

Not entirely sure if I did this correctly, I followed #573 as a template.

[Serial-ATA]

LGTM, just one correction on the ID3v2 mapping

[matteron]

I wasn't sure if this counted as a musicbrainz key or not. For future reference what delineates that a field is exclusively a MusicBrainz field?

[Serial-ATA]

AFAICT, there isn't really a "standard" field for release types outside of MusicBrainz. It looks like a lot of apps just make use of those MusicBrainz-specific fields (though I guess RELEASETYPE in Vorbis Comments could be generic). But now that you mention it, ItemKey::MusicBrainzReleaseType might be a better name just in case something else comes up.

[matteron]

Let me know if you want the description comment changed, I wasn't sure how exactly to explain it without just saying "The release type is the album's type"

[Serial-ATA]

Thanks! Updated the description a bit