[ie] Migrate commonly plural fields to lists (#8917)

Authored by: llistochek, pukkandan
Related: #3944

README.md

@@ -1311,7 +1311,8 @@ The available fields are:
  - `display_id` (string): An alternative identifier for the video
  - `uploader` (string): Full name of the video uploader
  - `license` (string): License name the video is licensed under
- - `creator` (string): The creator of the video
+ - `creators` (list): The creators of the video
+ - `creator` (string): The creators of the video; comma-separated
  - `timestamp` (numeric): UNIX timestamp of the moment the video became available
  - `upload_date` (string): Video upload date in UTC (YYYYMMDD)
  - `release_timestamp` (numeric): UNIX timestamp of the moment the video was released
@@ -1385,11 +1386,16 @@ Available for the media that is a track or a part of a music album:
  - `track` (string): Title of the track
  - `track_number` (numeric): Number of the track within an album or a disc
  - `track_id` (string): Id of the track
- - `artist` (string): Artist(s) of the track
- - `genre` (string): Genre(s) of the track
+ - `artists` (list): Artist(s) of the track
+ - `artist` (string): Artist(s) of the track; comma-separated
+ - `genres` (list): Genre(s) of the track
+ - `genre` (string): Genre(s) of the track; comma-separated
+ - `composers` (list): Composer(s) of the piece
+ - `composer` (string): Composer(s) of the piece; comma-separated
  - `album` (string): Title of the album the track belongs to
  - `album_type` (string): Type of the album
- - `album_artist` (string): List of all artists appeared on the album
+ - `album_artists` (list): All artists appeared on the album
+ - `album_artist` (string): All artists appeared on the album; comma-separated
  - `disc_number` (numeric): Number of the disc or other physical medium the track belongs to
 
 Available only when using `--download-sections` and for `chapter:` prefix when using `--split-chapters` for videos with internal chapters:
@@ -1767,10 +1773,11 @@ Metadata fields            | From
 `description`,  `synopsis` | `description`
 `purl`, `comment`          | `webpage_url`
 `track`                    | `track_number`
-`artist`                   | `artist`, `creator`, `uploader` or `uploader_id`
-`genre`                    | `genre`
+`artist`                   | `artist`, `artists`, `creator`, `creators`, `uploader` or `uploader_id`
+`composer`                 | `composer` or `composers`
+`genre`                    | `genre` or `genres`
 `album`                    | `album`
-`album_artist`             | `album_artist`
+`album_artist`             | `album_artist` or `album_artists`
 `disc`                     | `disc_number`
 `show`                     | `series`
 `season_number`            | `season_number`