The `definitions` field in `JSONSchemaObject` has the wrong type and JSON name

[smoyer64]

Describe the bug
The definitions field in JSONSchemaObject has the wrong type and JSON name. According to the jsonschema specification, this type (given the meta-schema code) should be map[string]JSONSchema and when marshaled to JSON, the object's field should be $defs. See https://www.learnjsonschema.com/2020-12/core/defs/.

To Reproduce
Add one or more definitions to a JSONSchema and marshal to JSON.

Expected behavior
The output should conform to the specification and should be reported as a valid JSON schema. The definitions field is named "definitions" instead of "$defs" and validation fails (or the definitions field is considered additional properties).

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Additional context
This is clearly a breaking change but one is needed to comply with the specifications. Assign this to me if you want me to make the change.

[jeremyfiel]

definitions is the correct attribute name for JSON Schema draft-07. $defs was only introduced in later drafts.

https://www.learnjsonschema.com/draft7/validation/definitions/