plugins.youtube: add short video URLs

[bastimeyer]

Resolves #3676

$ streamlink -l debug https://youtu.be/SebJIjQgdjg
[cli][debug] OS:         Linux-5.12.0-rc5-1-git-x86_64-with-glibc2.33
[cli][debug] Python:     3.9.2
[cli][debug] Streamlink: 2.1.1+4.g303c424
[cli][debug] Requests(2.25.1), Socks(1.7.1), Websocket(0.58.0)
[cli][debug] Arguments:
[cli][debug]  url=https://youtu.be/SebJIjQgdjg
[cli][debug]  --loglevel=debug
[cli][debug]  --player=mpv
[cli][info] Found matching plugin youtube for URL https://youtu.be/SebJIjQgdjg
[plugins.youtube][debug] Video ID from URL
[plugins.youtube][debug] Using video ID: SebJIjQgdjg
[plugins.youtube][debug] get_video_info - 1: Found data
[plugins.youtube][debug] This video is live.
[utils.l10n][debug] Language code: en_US
Available streams: 144p (worst), 240p, 360p, 480p, 720p, 1080p (best)