Expose _Wildcard as a public class for type hinting

Hi, with recent improvements to typing introduced in dash 3.0, we are looking into introducing pyright into our repo, and one of the problems we encountered is that `_Wildcard` is not exposed as a public class, which prevents us from properly type hinting our AiO components.

`NoUpdate` has the same issue, but there is already an open issue for that #2799.