declare var for WebIDL interface should always have new ()

[kainino0x]

declare var for WebIDL interfaces without constructors currently omit the new () definition:

declare var GPUDevice: {
  prototype: GPUDevice;
};

However, this prevents expressions like mydevice instanceof GPUDevice from being valid.
A new () method should be added to the generated var.

Upstream, TypeScript's built-in types would do this:

declare var GPUDevice: {
  prototype: GPUDevice;
  new (): GPUDevice;
};

However, if there is no constructor present in the IDL, then the constructor exists but will always throw. Hence, I think the generator could optionally write:

declare var GPUDevice: {
  prototype: GPUDevice;
  new (): never;
};

to be more precise.