Add UWP .NET 9 support for MSTest extensions

[Sergio0694]

This PR includes the initial work to support UWP apps using .NET 9 (see here).
This tackles the extensions assembly:

• Multi-target net8.0-10.0.26100.0 (we can bump to .NET 9 once it goes GA)
• Define WINDOWS_UWP on this target
• Pack this .dll into net8.0-windows10.0.26100.0/uwp
• Update .targets to reference it if UseUwpTools is set (ie. this is a UWP app using .NET 9)

[Youssef1313]

Suggested change

	<!-- net9.0 -->
	<!-- net9.0-windows10.0.17763.0 -->

[Youssef1313]

@Sergio0694 Looks like there is a packaging failure.

D:\a\_work\1\s\.dotnet\sdk\9.0.100\Sdks\NuGet.Build.Tasks.Pack\buildCrossTargeting\NuGet.Build.Tasks.Pack.targets(221,5): error NU5050: Attempted to pack multiple files into the same location(s). The following destinations were used multiple times: contentFiles\any\any\Extensions\WinUI\Microsoft.VisualStudio.TestPlatform.TestFramework.Extensions.xml [D:\a\_work\1\s\src\Package\MSTest.Internal.TestFx.Documentation\MSTest.Internal.TestFx.Documentation.csproj]
##[error].dotnet\sdk\9.0.100\Sdks\NuGet.Build.Tasks.Pack\buildCrossTargeting\NuGet.Build.Tasks.Pack.targets(221,5): error NU5050: (NETCORE_ENGINEERING_TELEMETRY=Build) Attempted to pack multiple files into the same location(s). The following destinations were used multiple times: contentFiles\any\any\Extensions\WinUI\Microsoft.VisualStudio.TestPlatform.TestFramework.Extensions.xml

[Evangelink]

It's ok we will sync with @Sergio0694 offline this week to answer a couple of high level questions before we resume work here.

Thanks @Youssef1313