Update Model Loading API to 1.21.4

[PepperCode1]

• Split model modifier events and callbacks - one set for static models and one set for block models
  • This is necessary because static models use UnbakedModel and are baked with settings while block models use GroupableModel and are not baked with settings
  • This cleans up the Identifier/ModelIdentifier getters
  • This allows providing the BlockState directly to ModelModifier.OnLoadBlock
• Remove before and after bake events
  • BeforeBake and AfterBake can be perfectly replicated with OnLoad since it is now possible to replace any UnbakedModel with any other UnbakedModel (not just JsonUnbakedModel) in 1.21.4
  • BeforeBakeBlock and AfterBakeBlock can be perfectly replicated with OnLoadBlock and GroupableModel no longer extends UnbakedModel
  • These events were given a Baker, but ran after model discovery was done, so using the baker could be unreliable
  • BeforeBakeBlock and AfterBakeBlock run after the creation of block model groups is done, so replacing the block model using these events would not update the block model groups
• ModelModifier.OnLoad can now accept a null model, which is a model that was requested during resolution but does not have a corresponding JSON file; Remove ModelResolver as its functionality can now be perfectly replicated with ModelModifier.OnLoad with OVERRIDE_PHASE
• Remove sprite getter from callback contexts as it is now accessible directly through the Baker
• Adjust Nullable annotations to match vanilla changes
• Add UnwrappableBakedModel
  • Moved and renamed from FRAPI's WrapperBakedModel (original will be deleted in Port FRAPI to 1.21.4 and refactor #4247)
  • Implement it and interface inject it on vanilla's WrapperBakedModel
  • Add new static UnwrappableBakedModel#unwrap method which accepts a Predicate saying when to stop unwrapping
• Add WrapperUnbakedModel which simply delegates all method calls to a different UnbakedModel
• Add WrapperGroupableModel which simply delegates all method calls to a different GroupableModel
• Remove DelegatingUnbakedModel as GroupableModel no longer extends UnbakedModel, so BlockModelResolvers can no longer use it, which was its original purpose; it is also not suitable for use in ModelModifier.OnLoad as returning one would affect all child models, which is usually not the desired behavior
• Remove BuiltinItemRenderer, BuiltinItemRendererRegistry, and BuiltinItemRendererRegistryImpl (part of Rendering v1, not Model Loading API v1) as they were replaced by a TAW to SpecialModelTypes.ID_MAPPER