Add $defs and $anchor support to JSONSchemaConverter

[criccomini]

#343 added basic $id and $ref support, but there's a lot more to add. The current implementation doesn't support $defs.

https://json-schema.org/draft/2020-12/json-schema-core.html#name-dereferencing

Update the converter to support $defs and and properly dereference $anchor refs.