Add Node interface codegen functionality

There is a known limitation where if you have types implementing the node interface you have to manually define the [typenames again](https://github.com/captbaritone/grats/blob/main/examples/production-app/graphql/Node.ts#L46-L57).

As a result, this loses type safety and can be easy to miss.

I'm not too sure what the typescript compiler has access to but potentially there could be a codegen step to generate a schema-types.ts with all the typenames that implement node.

Potentially it may require having a special `@gqlNodeInterface` comment tag in the application code

Start with examples/production-app/graphql/Node.ts, especially the manually repeated typenames, and inspect how the TypeScript compiler and proposed code-generation step could discover Node implementations. Review the idea of generating schema-types.ts and the possible @gqlNodeInterface comment tag. Done means Node implementations no longer require manually duplicated typenames while retaining type safety.

Written by the indexing model from the issue text.