Adding `title` and `description` to JSON Schema for Fields

[madelyneriksen]

I'm using JSON schema with React JSONSchema Forms to create forms automatically.

Currently, typesystem doesn't emit metadata like title or description into the rendered schema for fields (or schema objects). It does use those parameters for rendering forms, so I think a case can be made for adding it to the generated json schema too if present.

I'd be happy to open a PR for this, if it's behaviour that typesystem can or should have. I've already added this functionality to a vendored version. As far as I can tell, all that's required is adding checks to typesystem/json_schema.pyto the serialization function. I'm not sure if we'd want the same functionality in the de-serialization function.

P.S. Thank you for the great library!