plugins.nbcnews: fix plugin

[bastimeyer]

Closes #4664

Cleanup of the issues mentioned in my review of #4664, with a plugin rewrite

$ streamlink -l debug nbcnews.com/now
[cli][debug] OS:         Linux-5.18.11-1-git-x86_64-with-glibc2.35
[cli][debug] Python:     3.10.5
[cli][debug] Streamlink: 4.2.0+15.g20db6378
[cli][debug] Dependencies:
[cli][debug]  isodate: 0.6.1
[cli][debug]  lxml: 4.9.0
[cli][debug]  pycountry: 22.3.5
[cli][debug]  pycryptodome: 3.14.1
[cli][debug]  PySocks: 1.7.1
[cli][debug]  requests: 2.28.1
[cli][debug]  websocket-client: 1.3.2
[cli][debug] Arguments:
[cli][debug]  url=nbcnews.com/now
[cli][debug]  --loglevel=debug
[cli][debug]  --player=mpv
[cli][info] Found matching plugin nbcnews for URL nbcnews.com/now
[plugins.nbcnews][debug] API ID: 2007524
[utils.l10n][debug] Language code: en_US
Available streams: 144p_alt (worst), 144p, 216p_alt, 216p, 288p_alt, 288p, 360p_alt, 360p, 504p_alt, 504p, 576p_alt, 576p, 720p_alt, 720p, 1080p_alt, 1080p (best)