[Enhancement] Add DefaultView Property to WPF/Uno Platform

Summary

Prism.Maui has a DefaultView property for the RegionManager this allows you to simultaneously declare a region and set it's default view from XAML without the need to define this in C# code. There is no matching API for WPF and Uno Platform, this should be added to ensure API alignment and provide a better experience for those developing WPF and Uno Platform apps.