feat: support the other def keyword

[vetsin]

To address #42 -- it does not fully support the json pointer spec as it 'should' perhaps, but it works for the common $defs/definitions use case.

[jiro4989]

Sorry, I'm away for a few days so I'll check when I get back.

[jiro4989]

Hmm, I guess this is the only way to do it now. I couldn't implement #42 because of I couldn't support dynamic definition in Nim.

I think supporting only common terms is a practical solution.

[jiro4989]

OK

(*'-')! < % cat examples/json_schema_2.json
{
  "$id": "https://example.com/product.schema.json",
  "type": "object",
  "properties": {
    "product": { "$ref": "#/$defs/product" },
    "testproduct": { "$ref": "#/definitions/testproduct" }
  },
  "$defs": {
    "product": { "type": "string" }
  },
  "definitions": {
    "testproduct": { "type": "array", "items": { "type": "string" } }
  }
}

(*'-')! < % ./bin/nimjson -j ./examples/json_schema_2.json
type
  Object = ref object
    product: Option[Product]
    testproduct: Option[Testproduct]
  Product = string
  Testproduct = seq[string]

[jiro4989]

@vetsin Sorry I'm late.

LGTM 👍