[extractor/sibnet] Separate from VKIE

Fixes bfd973ece3 (commitcomment-91834251)

yt_dlp/extractor/_extractors.py

@@ -1639,6 +1639,7 @@ from .shared import (
     VivoIE,
 )
 from .sharevideos import ShareVideosEmbedIE
+from .sibnet import SibnetEmbedIE
 from .shemaroome import ShemarooMeIE
 from .showroomlive import ShowRoomLiveIE
 from .simplecast import (