Add a consult-emms-library source for playlists

[Hugo-Heagren]

(As in stored playlists which can be added with emms-source-playlist.) Might be good ot have a variable dictating where these are generally kept -- a defcustom like consult-emms-playlist-dir, default value wuld just be to inherit from emms-directory. Better yet -- propose that EMMS itself have such a variable, and submit a patch for it, then use it here.