Support JSON-Schema

It should be possible to serialize / deserialize types to / from JSON-Schema. That is -

Given a JSON Schema object I should be able to turn it into flow runtime `Type` instances (which can then be turned into static type definitions via `.toString()`).

Given some `Type`s, I should be able to generate a JSON Schema document.

I don't know if it will be possible to do this "losslessly" as JSON Schema includes some meta data that we don't e.g. labels and descriptions, but things like validation rules should be transferable as the validators in `flow-runtime-validators` are based on `revalidator` which uses JSON Schema.

cc @STRML because I think you were looking into this a while ago?