Share System.Private.Windows.Core with WPF

[JeremyKuhne]

This core assembly has no dependencies. It provides helpful interop utilities and would allow sharing common functionality such as clipboard implementations.

As it is already available for WPF compilation, the initial plan is to add internals visible to for WPF assemblies and just consume it.

Vetting can be done by updating dotnet/wpf#7929 to use the shared code.

We'll likely want to move more CsWin32 imports and helper code down from System.Windows.Forms.Primitives.