pema: add `JSONType`

[terrablue]

Validate that a value is a JSONValue according to @rcompat/type/JSONValue.
Will be a Storeable and map to a new json DataType (some databases have dedicate columns for JSON; in document-based databases, this can be embedded; otherwise, store as a JSON string in a text column).

Prototyping:

export default class JSONType
  extends PrimitiveType<object, "JSONType">
  implements Storeable<"json"> {

  get datatype() {
    return "json" as const;
  }
}