Completely rewrite how subscription validation is performed. Optimizes it quite a bit while also adding backend support for the upcoming `dissect` sub-command, where you can resolve any subscription into its 'raw' form for easier debugging when making scripting changes.
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
Reduce memory usage significantly when pulling large channels all at once by
- Not loading entry script until it is being processed
- Deleting the entry memory once fully processed
- Disable yt-dlp from keeping entry info in memory since ytdl-sub reads it from file
