Add pl_thumbnail outtmpl key for playlist thumbnails

This should have been implemented in 681de68e9d, but I forgot

yt_dlp/utils.py

@@ -4311,6 +4311,7 @@ OUTTMPL_TYPES = {
     'description': 'description',
     'annotation': 'annotations.xml',
     'infojson': 'info.json',
+    'pl_thumbnail': None,
     'pl_description': 'description',
     'pl_infojson': 'info.json',
 }