plugins.gulli: rewrite and fix plugin

[bastimeyer]

Fixes #5724

@gkovacsp if you want, please give these changes a try and report back if anything is not working. Thanks. Instructions here:
https://github.com/streamlink/streamlink/blob/master/CONTRIBUTING.md#pull-request-feedback

$ HTTPS_PROXY=socks5h://localhost:1920 ./script/test-plugin-urls.py gulli
:: Finding streams for URL: https://replay.gulli.fr/Direct
:: Found streams: 224p, 360p, 404p, 540p, worst, best
:: Finding streams for URL: https://replay.gulli.fr/dessins-animes/Bob-l-eponge-s10/bob-l-eponge-s10-e05-un-bon-gros-dodo-vod69736581475000
:: Found streams: 404p, worst, best
:: Finding streams for URL: https://replay.gulli.fr/emissions/Animaux-VIP-une-bete-de-reno-s02/animaux-vip-une-bete-de-reno-s02-e09-la-taniere-du-dragon-vod69634261609000
:: Found streams: 404p, worst, best
:: Finding streams for URL: https://replay.gulli.fr/series/Black-Panther-Dangers-au-Wakanda/black-panther-dangers-au-wakanda-black-panther-dangers-au-wakanda-vod69941412154000
:: Found streams: 404p, worst, best

[gkovacsp]

It does work, thank you!