Support conversion of cyclic types to JSON Schema

[ssalbdivad]

This would allow types like the following to be converted to JSON schema via $ref:

import { scope } from "arktype";

const types = scope({ a: { b: "b" }, b: { a: "a" } }).export();

const jsonSchema = types.a.toJsonSchema();

See https://json-schema.org/understanding-json-schema/structuring