VS: DOTNET_SDK support for core msbuild attributes

In a recent issue on DOTNET_SDK support, @rainersigwald proposed looking at core MSBuild attributes. This issue tracks the support for each of those attributes. Below is a slight edit of

• VS_DOTNET_DOCUMENTATION_FILE
• VS_DOTNET_REFERENCE_
• VS_DOTNET_REFERENCEPROP_
• VS_DOTNET_REFERENCES (ideally only when targeting .NET Framework and error if targeting .NET Core/.NET 5.0+)
• VS_PACKAGE_REFERENCES
• DOTNET_TARGET_FRAMEWORK
• VS_GLOBAL_ROOTNAMESPACE
• VS_GLOBAL_<variable>

Additionally, I think it does not make sense to support

• DOTNET_TARGET_FRAMEWORK_VERSION, since that's tied to .NET Framework and shouldn't be used for .NET Core/.NET 5.0+ projects (prefer DOTNET_TARGET_FRAMEWORK's specification of a target framework moniker instead)
• VS_PROJECT_IMPORT, since imports from NuGet packages should be handled by dotnet restore/MSBuild.exe -restore

There are also a couple of new things to consider supporting:

• Runtime identifier, the .NET Core/.NET 5.0+ mechanism for supporting target OS/OS version/architecture. For RuntimeIdentifier, CMake supports it via VS_GLOBAL_RuntimeIdentifier. I am hesitant adding another variable.

FYI @baronfel