Adds the ability to make thumbnails square, both file and embedded. Usage:
```
my_preset:
square_thumbnail: True
```
Will soon enable this by default for all music-based presets with a toggle to disable.
Huge thanks to @Kentaro1043 for crafting the ffmpeg command for this!
Closes https://github.com/jmbannon/ytdl-sub/issues/383
A complete gutting of the internals of ytdl-sub to support functions in our variable syntax, in addition to being able to access a yt-dlp entry's .info.json fields using functions. Functionally, ytdl-sub should still look and behave the same from a user-perspective.
With so many lines of code changed (+8927, -2708), no doubt there will be new issues. Please make a GH issue or reach out on Discord if your config/subscriptions break in any way/shape/form.
Details on how to use function support will come soon in the form of proper documentation in our readthedocs.
* [BUGFIX] Do not fail with thumbnail issues
* helper
* refactor
* dry-run
* regen fixture
* download arhcive txt -> json in docs, fixtures
* return if no download thumb path
* better test
* more test coverage
