Build MSI installers for targeting pack (#7726)

Part of #6501 

This adds a new Windows installer for the targeting pack. It places *.dll and *.xml (docs) in `[DOTNETHOME]\packs\Microsoft.AspNetCore.App.Ref\$(version)\ref\netcoreapp3.0`.

Outputs:
* aspnetcore-targeting-pack-$(version).zip
* aspnetcore-targeting-pack-$(version)-win-x64.exe (defaults to C:\Program Files\dotnet)
* aspnetcore-targeting-pack-$(version)-win-x86.exe (defaults to C:\Program Files (x86)\dotnet)

These all include the same files. These are meant to be bundled in the .NET Core SDK installer, but can be launched directly too.