From https://github.com/metabrainz/picard/blob/8245afbd5ba1f345e7ab61db83dc7af67e6f482c/picard/mbjson.py#L63, it looks like Picard already has a ~musicbrainz_tracknumber variable. Would it be possible to get that as a real tag by default please?

Background: I'm writing a new music player, and I'd like to use that tag to make tracklists prettier.