Support $ref along with definitions

[ostrowr]

Right now, schemas that use $ref get an unknown type.

Dealing with JSON Pointer through the type system seems like it will be a nightmare.

Maybe it's possible to enforce that the $id field is required in definitions, and that each ref points to one of those ids? Then we would resolve the right type without too much trouble.

[ostrowr]

https://json-schema.org/understanding-json-schema/structuring.html#using-id-with-ref