handle default export in module augmentation

[jamescdavis]

The Glint transform currently trips on a default export in a module augmentation included in a component class file, e.g. augmenting the component registry:

import Component from '@glimmer/component';

export default class FooComponent extends Component {}

declare module '@glint/environment-ember-loose/types/registry' {
  export default interface Registry {
    Foo: FooComponent;
  }
}

This adds a failing test for this and then adds handling of it by preventing traversal into declare module.

[dfreeman]

Nice catch—thanks for the fix! It looks like we've got some trouble with TS nightly right now; I'll see if I can't get that green so we can merge this

[dfreeman]

Ok, all fixed on master!