Clean up mixins

[Daomephsta]

Below is a list of the changes with justification and an explanation for readers of the repo wondering why the code differs from the video.

• Replace ItemRendererAccessor with a @Shadow in ItemRendererMixin. A shadow is a simpler and cleaner way to access a field declared by the mixin's target class. Accessors are for inaccessible fields of other classes.
  The shadow is @Final rather than final because
• Replace ModelLoaderMixin with a ModelLoadingPlugin. FAPI APIs are preferable to mixins, as FAPI will avoid changing APIs if possible, while the internals mixins depend on may change at any time.
• Make handler methods private when possible. It's standard Java best practice to minimise access levels. This applies to mixins too,
• Use a constant ModelIdentifier field for the ruby staff. Avoids a typo in the mixin or ModelLoadingPlugin breaking the 3d model.