Support generics and/or overrides.

[jmoseley]

Currently the following code will error with Error: Don't know how to handle AnyKeyword nodes:

// Foo.ts
export interface Foo {
  prop:any;
}

// schema.ts
import Foo from './foo';

export interface Bar extends Foo {
  prop:SomeConcreteType;
}

The expectation is that the GQL would contain the type:

type Bar {
  prop: SomeConcreteType
}

Alternatively the source code could be written:

// Foo.ts
export interface Foo<T> {
  prop:T;
}

// schema.ts
import Foo from './Foo';
 
export interface Bar extends Foo<SomeConcreteType>;

And that would result in the same output.