[boosty] added new direct message extractor

- formatting
- fixed linting formatting errors
- fixed E999 SyntaxError: invalid syntax
- fixed class naming
- fixed mandatory extractor.boosty.metadata as true requirement
- update
  - apply changes
  - add test
  - update docs/supportedsites
- improve 'dialog' pagination logic

scripts/supportedsites.py

@@ -211,6 +211,7 @@ SUBCATEGORY_MAP = {
         "posts": "",
     },
     "boosty": {
+        "direct-messages": "DMs",
         "feed": "Subscriptions Feed",
     },
     "civitai": {