sdk: auto-detect StrideNativeBuildMode

[xen2]

Pick MSVC only when targeting Windows AND the VS C++ tools are loaded
(VCINSTALLDIR set), Clang otherwise.

• Plain dotnet build from a regular shell on Windows now works out of the box
• Cross-compile from Windows host (Linux/Android/iOS targets) auto-uses Clang
• Drops the now-redundant -p:StrideNativeBuildMode=Clang from many CI workflows (release.yml keeps it, it runs msbuild.exe
  with VS env active)

Note: we'll have to check if MSVC mode is really necessary still at all.

[Kryptos-FR]

@xen2 dotnet build will fail when used from within a Visual Studio Prompt because dotnet is not able to build C++ projects. It still needs to use msbuild.

C:\Projects\Stride\Engine\stride\sources\native\WindowsProjects\WindowsDesktop\WindowsDesktop.vcxproj(35,3): error MSB4278: The imported file "$(VCTargetsPath)\Microsoft.Cpp.Default.props" does not exist and appears to be part of a Visual Studio component. This file may require MSBuild.exe in order to be imported successfully, and so may fail to build in the dotnet CLI.
Stride.Audio net10.0 failed with 10 warning(s) (1.7s)

Still, it is a big improvement because now dotnet build does work from a regular console prompt. We need to update our build documentation to explain this behavior.

[xen2]

@xen2 dotnet build will fail when used from within a Visual Studio Prompt because dotnet is not able to build C++ projects. It still needs to use msbuild.

C:\Projects\Stride\Engine\stride\sources\native\WindowsProjects\WindowsDesktop\WindowsDesktop.vcxproj(35,3): error MSB4278: The imported file "$(VCTargetsPath)\Microsoft.Cpp.Default.props" does not exist and appears to be part of a Visual Studio component. This file may require MSBuild.exe in order to be imported successfully, and so may fail to build in the dotnet CLI.
Stride.Audio net10.0 failed with 10 warning(s) (1.7s)

Still, it is a big improvement because now dotnet build does work from a regular console prompt. We need to update our build documentation to explain this behavior.

Thanks for the feedback, I reviewed all case and I think the 55b468e should fix those.

I also took the opportunity to simplify top-level README.md build instructions and moved details to another file in 6ee2e04.
This also include a workaround so that even if ARM64 build tools are not installed, it works for local dev. VS with 2 simple workload and everything should work out of the box now! (as soon as we recompile that .NET 6 dependency in deps)