[ENHANCEMENT] Better mod support for stickers (StickerRegistry)

[AbnormalPoof]

Does this PR close any issues? If so, link them below.

Implements #3969

Briefly describe the issue(s) fixed.

This PR moves sticker sets to its own registry called StickerRegistry, and replaces the old (and quite frankly crude) StickerInfo class with a new class called StickerSet. This should hopefully streamline the process of making custom sticker sets, since mods no longer need to directly replace stickers-set-1 or use JSON patching.

Notes

• This could be considered a breaking change, since JSON files for sticker sets now reside in data/stickersets (and StickerInfo being removed). But that's up to Eric to decide.
• I've created a pull request on the funkin.assets repository to update stickers-set-1: Move stickers-set-1 to data/stickersets/ funkin.assets#109
• StickerSprite now takes a file path instead of sticker set and sticker names to support custom file paths for sticker sets.

[ChillyBeanBAM]

Oh, this is nice! Good work!

[EliteMasterEric]

I like how this standardizes the stickers to match how the rest of the game uses data.

However, this PR has merge conflicts with v0.6 (it utilizes the previously unused sticker pack system). We can resolve those at that point.