Issue 11706 - Add a TypedefType trait to extract the underlying type of a std.typecons.Typedef

https://issues.dlang.org/show_bug.cgi?id=11706

Thanks to some compiler improvements this can be entirely implemented outside the `Typedef` definition.
Oh you mean `= Args` rather than `= Args[0]`? I can't remember why I used `[0]` here, should be fixed.