Use type for empty interfaces.

[Bashamega]

It would reduce the size of the generated content if empty interfaces were transformed into type aliases, like this:
https://github.com/microsoft/TypeScript-DOM-lib-generator/blob/246360bbfcc64d3d28a772d77edfbdc69cb22747/baselines/dom.generated.d.ts#L1634
To:

type RTCAnswerOptions = RTCOfferAnswerOptions;

We can update the emitter to do this.

This might also slightly reduce the size of the shipped lib.dom.d.ts file in the main TypeScript repo, since type aliases are shorter to declare than empty interfaces. It’s probably a small saving, but still a nice cleanup.
I can work on this

[jakebailey]

I don't understand why this is a good idea; interfaces and aliases have different semantics, especially for global ones that need merging.

[Bashamega]

For now it doesn’t have any properties. If we ever need to add some in the future, the emitter could switch it back to an interface.

My thought was: before emitting, check whether the declaration has any properties/methods/events; if not, emit it as a type alias instead.

But since some developers might extend them maybe it is safe to keep it as is