Write thumbnail of playlist

Related: https://github.com/ytdl-org/youtube-dl/pull/28872, https://github.com/ytdl-org/youtube-dl/pull/28860
This is slightly different from the above PRs in that this downloads the playlist's thumbnail instead of the uploader's profile picture. But for youtube channel URLs these are the same

yt_dlp/YoutubeDL.py

@@ -1379,6 +1379,9 @@ class YoutubeDL(object):
                     except (OSError, IOError):
                         self.report_error('Cannot write playlist metadata to JSON file ' + infofn)
 
+            # TODO: This should be passed to ThumbnailsConvertor if necessary
+            self._write_thumbnails(ie_copy, self.prepare_filename(ie_copy, 'pl_thumbnail'))
+
             if self.params.get('writedescription', False):
                 descfn = self.prepare_filename(ie_copy, 'pl_description')
                 if not self._ensure_dir_exists(encodeFilename(descfn)):