Replace type definition being imported by `repository-json-schema`

[shimks]

Description / Steps to reproduce / Feature proposal

We use typescript-json-schema module to provide us with type definitions for the JSON schemas we generate in repository-json-schema, but we don't use any of its other functionalities which use typescript itself as a dependency. typescript is too huge to be exported over whenever we want to generate JSON schemas so we want to replace them with something lighter.

Acceptance Criteria

• replace interface JsonDefinition with JSONSchema6 from @types/json-schema https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/json-schema/index.d.ts#L216
• potentially rename JSONSchema6 to JSONSchema for re-export
• fix any mention of JsonDefinition in the tests, docs, readme, etc.
  • NOTE: there may be compilation errors that arise from the switch, and any changes in the docs/README should be tested to make sure there are no compilation errors.

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