Add `pl_thumbnail` outtmpl key for playlist thumbnails

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

README.md

@@ -885,7 +885,7 @@ To summarize, the general syntax for a field is:
 %(name[.keys][addition][>strf][|default])[flags][width][.precision][length]type
 ```
 
-Additionally, you can set different output templates for the various metadata files separately from the general output template by specifying the type of file followed by the template separated by a colon `:`. The different file types supported are `subtitle`, `thumbnail`, `description`, `annotation`, `infojson`, `pl_description`, `pl_infojson`, `chapter`. For example, `-o '%(title)s.%(ext)s' -o 'thumbnail:%(title)s\%(title)s.%(ext)s'`  will put the thumbnails in a folder with the same name as the video.
+Additionally, you can set different output templates for the various metadata files separately from the general output template by specifying the type of file followed by the template separated by a colon `:`. The different file types supported are `subtitle`, `thumbnail`, `description`, `annotation`, `infojson`, `pl_thumbnail`, `pl_description`, `pl_infojson`, `chapter`. For example, `-o '%(title)s.%(ext)s' -o 'thumbnail:%(title)s\%(title)s.%(ext)s'`  will put the thumbnails in a folder with the same name as the video.
 
 The available fields are:
 

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',
 }