command: add playlist-next-playlist and playlist-prev-playlist

playlist-prev-playlist goes to the beginning of the previous playlist
because this seems more useful and symmetrical to
playlist-next-playlist. It does not go to the beginning when the current
playlist-path starts with the previous playlist-path, e.g. with mpv
--loop-playlist foo/, which expands to foo/{1..9}.zip, the current
playlist path foo/1.zip beings with the playlist-path foo/ of {2..9}.zip
and thus playlist-prev-playlist goes to 9.zip rather than to 2.zip.

Closes #12495.