[extractors] Use new framework for existing embeds (#4307)

`Brightcove` is difficult to migrate because it's subclasses may depend
on the signature of the current functions. So it is left as-is for now

Note: Tests have not been migrated

yt_dlp/extractor/libsyn.py

@@ -10,6 +10,7 @@ from ..utils import (
 
 class LibsynIE(InfoExtractor):
     _VALID_URL = r'(?P<mainurl>https?://html5-player\.libsyn\.com/embed/episode/id/(?P<id>[0-9]+))'
+    _EMBED_REGEX = [r'<iframe[^>]+src=(["\'])(?P<url>(?:https?:)?//html5-player\.libsyn\.com/embed/.+?)\1']
 
     _TESTS = [{
         'url': 'http://html5-player.libsyn.com/embed/episode/id/6385796/',