1.21.4 Resource Pack Support

[SpecialBuilder32]

Adds 1.21.4. item model definition files to the resource pack, and moves existing models to an overlay for backwards compatibility.

Remaining tasks:

• Testing in-game
• RP marked valid versions (1.21.3 should not throw warning)
• Restore backwards compatibility for shamir textures
• Fix vanilla item-model forwarding

[misode]

I'm getting lots of resource pack load errors.

[SpecialBuilder32]

I'm getting lots of resource pack load errors.

I was almost certain it did support item-model-definition compounds both when I tested it and re the wiki.

I'll do some more digging and see why I didn't get this problem on my end and/or what has changed

[misode]

@SpecialBuilder32 Have you had time to look further into these issues? Given that 1.21.5-pre1 just released, it would be ideal if this PR is ready to be merged before we have to worry with the 1.21.5 update.

[SpecialBuilder32]

In tracking down what the load errors were, I realized there were some special cases I needed to manually handle. I had thought it was just the end fishing elytra but now that I see build errors I realize armor trims must also be redone :/

[SpecialBuilder32]

Alright just a quick update on this - after running into too many edge cases, I'm half-rewriting the 1.20.4/1.20.5 update to make more sense and handle model-file/item-definition interactions better. It'll be a fair amount of more testing and debugging until this is ready. Sorry 😢