[web_generator] IDL Gen should use `JSAny?` instead of `JSVoid` for `void` types nested as a type parameter

[nikeokoronkwo]

The dart:js_interop interfaces containing type parameters expect that the type parameters extend JSAny. However, JSVoid does not extend JSAny, as it is just a typedef for void.

When IDL code is generated, this causes types in WebIDL like Promise<void> to become JSPromise<JSVoid>, which is not correct.

We should rather generate JSPromise as is, without the void type parameter where possible.

[srujzs]

I'm a little surprised we generate JSVoid but there isn't a precedent for Promise<void> in the webref. There aren't any mentions of JSVoid anywhere in the generator anymore.

[nikeokoronkwo]

I think it is because void in general does not appear that often as the webref used for generating package:web uses undefined instead for such cases where void could be used.
The addition of void was for inclusion in #377 so that it could incline with what the Dart JS Interop generator would put out.