Add WixToolset.Tool.nuget to build for feature #3981.

This change adds very raw support for NuGet packages to Simplified WiX and support for using Simplified WiX in the WiX Build process. All of that support allows us to create a WixToolset.Tools.swr to create the nuget package for the WiX v4.0 project.  Publishing the built nuget package will be a later change.