[feature] more filetype support!

[NyaaaWhatsUpDoc]

Description

This updates our code wrapping ffprobe / ffmpeg in the media processor to support more of the file types that the previously mentioned programs support decoding / encoding. This also updates the instance supported media mimetypes list to better reflect what we do support now (woohoo!), reduces the complexity of code in the media processing pipeline so audio / image / video types share as much code as possible, and converts the result output of ffprobe to a more useable data format.

Oh another nice little change, it tells ffprobe to use compact-json output so it reduces need for outputting so much whitespace. Not a massive change but i love ones like these as it's essentially free efficiency boost :)

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.