[ie] Add new fields with proper support for multiple values

yt_dlp/extractor/common.py

@@ -422,16 +422,23 @@ class InfoExtractor:
     track_number:   Number of the track within an album or a disc, as an integer.
     track_id:       Id of the track (useful in case of custom indexing, e.g. 6.iii),
                     as a unicode string.
-    artist:         Artist(s) of the track.
-    genre:          Genre(s) of the track.
+    artist_list:    Artist(s) of the track, as a list of unicode strings.
+    composer_list:  Composer(s) of the piece, as a list of unicode strings.
+    genre_list:     Genre(s) of the track, as a list of unicode strings.
+
     album:          Title of the album the track belongs to.
     album_type:     Type of the album (e.g. "Demo", "Full-length", "Split", "Compilation", etc).
-    album_artist:   List of all artists appeared on the album (e.g.
-                    "Ash Borer / Fell Voices" or "Various Artists", useful for splits
-                    and compilations).
+    album_artist_list: All artists appeared on the album, as a list of unicode strings.
+                    (e.g. ["Ash Borer", "Fell Voices"] or ["Various Artists"],
+                    useful for splits and compilations).
     disc_number:    Number of the disc or other physical medium the track belongs to,
                     as an integer.
-    composer:       Composer of the piece
+    composer:       Deprecated, use "composer_list" instead. Composer(s) of the piece,
+                    comma-separated
+    artist:         Deprecated, use "artist_list" instead. Artist(s) of the track, comma-separated.
+    genre:          Deprecated, use "genre_list" instead. Genre(s) of the track, comma-separated.
+    album_artist:   Deprecated, use "album_artist_list" instead. All artists appeared on the
+                    album, comma-separated.
 
     The following fields should only be set for clips that should be cut from the original video: