Add country release

[UnknownSuperficialNight]

Add ReleaseCountry to ItemKey

I propose adding ReleaseCountry to ItemKey. Please review to ensure I've covered everything.

This change adds ItemKey to a few key/ItemKey maps.

I have a question regarding ID3V2_MAP. Should it be:

"TXXX:MusicBrainz Album Release Country" => ReleaseCountry 
OR
"TXXX" => ReleaseCountry

I assume the TXXX:MusicBrainz Album Release Country one is correct?

I'm also unsure whether to map MusicBrainz Album Release Country directly to ReleaseCountry or create a separate MusicBrainzReleaseCountry. My assumption is to map it to a single ReleaseCountry.

Let me know if I missed anything, other, than those two.

Note-to-self:
Todo: ILST_MAP and ID3V2_MAP

[Serial-ATA]

I have a question regarding ID3V2_MAP. Should it be:

For keys that map to TXXX frames, you just put the description in there, no TXXX. So in this case, "MusicBrainz Album Release Country" => ReleaseCountry.

Then you just stick the ItemKey::ReleaseCountry in this list:

lofty-rs/lofty/src/id3/v2/tag.rs

Lines 1415 to 1423 in 0805a4c

	// Multi-valued TXXX key-to-frame mappings
	for item_key in [
	ItemKey::TrackArtists,
	ItemKey::Director,
	ItemKey::CatalogNumber,
	ItemKey::MusicBrainzArtistId,
	ItemKey::MusicBrainzReleaseArtistId,
	ItemKey::MusicBrainzWorkId,
	] {

Note that only ID3v2 has that special requirement, just because those handful of items map to a frame other than what their name specifies.

I'm also unsure whether to map MusicBrainz Album Release Country directly to ReleaseCountry or create a separate MusicBrainzReleaseCountry. My assumption is to map it to a single ReleaseCountry.

Yeah, it's unfortunate that they stuck "MusicBrainz" in there, but at the same time I believe it's the only standardized field for release country. I'd be fine just mapping it to ReleaseCountry, cause at the end of the day it's just a standard ISO 3166-1 code, typically.

And if it ever needs to change down the line, it can. No big deal.

[UnknownSuperficialNight]

@Serial-ATA Alright have a look if that's correct

[Serial-ATA]

Looks good, thanks for keeping the ilst map aligned as well. :)

Rebased and updated the changelog.