Fix tag marker preservation in list creation

[mikedilger]

See https://github.com/nostr-protocol/nips/pull/987#issuecomment-1892767097

I think the best thing to do is to change tags into a Vec<String>. Using a typed enum for Tags has bit me in the ass too many times.

This is a bug in that we don't preserve unknown future data, but it doesn't affect anybody yet AFAIK.