[extractor/twitch] Update thumbnail size

[JC-Chung]

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Currently, using --write-thumbnail when downloading vod will only give you a 90*60 size thumbnail.
like https://www.twitch.tv/videos/1773257171
https://static-cdn.jtvnw.net/cf_vods/d2nvs31859zcd8/3afded6c325c0f62cc4e_dashducks_41995346187_1679578217//thumb/thumb0-90x60.jpg
> https://static-cdn.jtvnw.net/cf_vods/d2nvs31859zcd8/3afded6c325c0f62cc4e_dashducks_41995346187_1679578217//thumb/thumb0-0x0.jpg
Use thumbnail = re.sub(r"(\d+)x(\d+)\.jpg", "0x0.jpg", thumbnail) to replace the thumbnail with size 0*0 (original size?)
And Live even though the highest picture quality is 1920*1080, currently only get 1280*720 thumbnail, using the same code modified to 0*0
like https://www.twitch.tv/dashducks
https://static-cdn.jtvnw.net/previews-ttv/live_user_dashducks-1280x720.jpg
> https://static-cdn.jtvnw.net/previews-ttv/live_user_dashducks-0x0.jpg

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Summary

🛠️📺🆗

Refactor twitch.py to use a new method for thumbnail extraction that handles fallbacks better.

We'll haul away the old code, me hearties, haul away
We'll make the thumbnails better, with a fallback today
We'll use the extract_thumbs method, on the count of three
We'll refactor Twitch videos, and live streams, yo ho heave

Walkthrough

• Add a new method _get_thumbnails to the TwitchBaseIE class that returns a list of two thumbnails with different resolutions for a given URL (link)
• Replace the thumbnail field with a thumbnails field in the info dictionary of the TwitchVodIE and TwitchStreamIE classes that use the _get_thumbnails method to provide a fallback thumbnail option (link, link)
• Remove the code that replaced the width and height parameters in the thumbnail URL with zeros in the TwitchVodIE class, as it is no longer needed (link)

[pukkandan]

Return both as thumbnails

[mauritzn]

Request change of overly static RegEx for thumbnail URL modification, to a more flexible RegEx.