Expose _Wildcard as a public class for type hinting

[tlauli]

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.