[webtoons] add 'thumbnails' option (#6468 #7441)

* Update webtoons.py
    Added thumbnail download capability which defaults to false
* Update configuration.rst
    Added documentation for webtoon thumbnail option
* extract thumbnails in GalleryExtractor.assets()
* simplify & fix flake8
* include 'type' in default filenames
* add test
* update docs

---------

Co-authored-by: Mike Fährmann <mike_faehrmann@web.de>

docs/gallery-dl.conf

@@ -785,7 +785,8 @@
         {
             "sleep-request": "0.5-1.5",
 
-            "quality": "original"
+            "quality"   : "original",
+            "thumbnails": false
         },
         "weebcentral":
         {