Support multi-valued tags

[deluan]

Hey @dhowden, have you considered supporting multi-valued tags? I started looking deeper in this package's codebase, and I think we could introduce it without breaking compatibility, by adding methods like

Values(name tag.Name) []string // this returns all values for a given tag, with a normalized (enum) name
Artists() []string
AlbumArtists() []string
Genres() []string
PictureTypes() []string
Pictures(type string) *Picture
RawValues() map[string][]any
...

Actually I just implemented Pictures/PictureTypes in my fork, to investigate/resolve an issue in my music server (navidrome/navidrome#3254).

Any thoughts? I can try to implement this if you feel it is a good addition.

By the way, here's my change implementing multiple pictures, but just for Vorbis for now: deluan@dfe5e6e