Implement KeyOf[T] type utility

[AliiiBenn]

Description

Add a KeyOf[T] type that extracts the literal keys of a type T, similar to TypeScript's keyof.

Specification

KeyOf[dict[str, int]] = Literal['keys', 'values', 'items', ...]
KeyOf[MyClass] = Literal['field1', 'field2', ...]
KeyOf[MyTypedDict] = Literal['key1', 'key2', ...]

Implementation notes

• Should work with classes, TypedDict, and regular dict types
• Needs runtime evaluator in typemap/type_eval/_eval_operators.py
• Should use existing introspection capabilities (Attrs, Members)
• Returns a union of Literal types

References

• PEP 827 draft mentions KeyOf as a requested feature (line 1666)
• TypeScript equivalent: keyof T

Acceptance criteria

• KeyOf works with regular classes
• KeyOf works with TypedDict
• KeyOf works with dict types
• Runtime evaluator passes all tests
• Unit tests added