[twitter] add 'user_likes' metadata field for liked tweets

i.e. the 'screen_name' of the user whose liked tweets get extracted.

Ideally this would replace 'user' or at least be in the same format,
but that would break backwards compatibility or be impossible/too
complicated thanks to API result differences.

(#1421)

gallery_dl/extractor/twitter.py

@@ -339,6 +339,9 @@ class TwitterLikesExtractor(TwitterExtractor):
     pattern = BASE_PATTERN + r"/(?!search)([^/?#]+)/likes(?!\w)"
     test = ("https://twitter.com/supernaturepics/likes",)
 
+    def metadata(self):
+        return {"user_likes": self.user}
+
     def tweets(self):
         return TwitterAPI(self).timeline_favorites(self.user)