[extractor/reddit] Add fallback format

[bashonly]

Some Reddit videos (hosted on v.redd.it) have a higher quality video format that was not being extracted.

fallback_url was already a field in the reddit_video JSON object: the extractor was previously using the value to find the video ID, but was not extracting the URL as a format. For some videos the fallback_url format's resolution can be up to 1080p, which is better than any formats found in the DASH MPD. I assume this is only the case with videos where the original source was 1080 or better.

Note: even though it is a direct http URL, the fallback_url is a video-only mp4 format, and Reddit seems to treat it like a DASH variant: the URL basename is usually DASH_1080.mp4 or DASH_{bitrate}.mp4

Fixes #5160

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