Publish a schema definition

[JonZeolla]

I'd like to be able to programmatically generate threat models in a deciduous.app compatible format.

[rpetrich]

A basic schema is now published as part of the types in https://github.com/rpetrich/deciduous/blob/master/layout.ts. TypeScript cannot 100% represent the precise type shapes, so consider SingleNamedProperty to describe a single arbitrarily-named property representing the ID of some node.