PlaylistItem.track possibly shouldn't be wrapped in Option

[kangalio]

At least when returned from Get Playlist, the field is never absent:

Link

I'm not sure which other endpoints return payloads containing PlaylistItem - if PlaylistItem.track is optional in any of those, we probably shouldn't duplicate PlaylistItem and instead add a doc comment to BaseClient.playlist(): "the [`PlaylistItem::track`] field in this response is always Some".

[github-actions]

Message to comment on stale issues. If none provided, will not mark issues stale

[kangalio]

Another valid inprovement suggestion lost to the relentless bug report steamroller called stale bot :'(