Expose the `ResponseStub` type

[IgnisDa]

I would like to use the ResponseStub to be able to extract a function that redirects.

[brophdawg11]

We're still considering some changes to the ResponseStub approach (https://github.com/remix-run/react-router/issues/11707, https://www.youtube.com/watch?v=f5z_axCofW0) so for now I would just duplicate the type locally

export type ResponseStub = {
  status: number | undefined;
  headers: Headers;
};

[brophdawg11]

This is no longer relevant as the response stub API has been removed