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.
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