Add a method to convert annotations to slots using metaclasses

[DavidCEllis]

• Add AnnotationsSlotsMeta metaclass to create slots based on annotations.
• Make Field subclasses no longer require @fieldclass
• Delete @fieldclass
• Convert @annotationclass decorator to AnnotationClass base class
  • This new base class supports __slots__ by default using the new metaclass.
• Redefine prefab's Attribute class using annotations to demonstrate that slots are created.

[DavidCEllis]

Python 3.14 is likely to make annotations more of a pain to work with, it might be necessary to move annotations related code into a submodule.