Rewrite Datasworn type composition/codegen as AST

[rsek]

Currently, Datasworn uses a set of functions to generate TS types from the JSON schema, and JTD for everything else. I'd like to phase out JTD as I think it's not meeting Datasworn's needs, particularly around class inheritance.

To replace it, I'm developing a Datasworn-specific AST that represents shapes common in Datasworn. These might be converted from JSON schema, or serve as a new "source of truth" that composes the JSON schema. Initially it will only generate TS and Java output, but the idea is to make it (relatively) easy to add code generation for any language.