demux_playlist: add support for 'strm' playlist

The .strm format consists of a URI or local path as the sole contents
of a text file, this is parsed by software like Emby or Jellyfin to
treat remote streams or paths as if they are in the local filesystem.
This commit adds .strm to mpv's internal list of playlist file formats,
and points it to the handler for .m3u files as they should be
compatible. .strm doesn't need all the features of .m3u/.m3u8, but the
features do not cause issues with the relatively barebones playlist
format of .strm.