Clean up and modernise the JSON Schema for the spec #69

I would very much like to clean up and modernise the JSON Schema for the spec to bring it to Draft 2020-12 (which prepares it for JSON Schema 1.0).

This must not create any semantic changes, but should, for example, pull out common subschema that are currently inlined in the main body of the schema. This significantly helps tooling with naming and code generation.

JSON that validates/fails to validate against the Draft-7 schema should also validate/fail to validate against the refactored schema. Simple tests should be created from a body of real world examples to ensure that common schema still validate correctly.

It should be possible to maintain both Draft 7 and Draft 2020-12 Schema with Draft 7 'deprecated', moving to Draft 2020-12 with any later version of the spec.

For what it is worth, I'm on the TSC for JSON Schema, and I think this would be a good case study for JSON Schema and tooling.