Small typing improvement for AsyncCollectionReference and AsyncQuery

[sergiterupri]

Is your feature request related to a problem? Please describe.
Typing isn't working well when using AsyncCollectionReference and AsyncQuery. As an example, where method which is inherited on both cases, will "return" a BaseQuery.

Having an AsyncQuery:

After using where the editor thinks it is a BaseQuery:

And because of this the editor says I can't do "async stuff":

Describe the solution you'd like
Correct typing for AsyncCollectionReference and AsyncQuery methods.

Additional context
I'm also providing a small PR with required changes.