Make standalone v2's OS | Arch combination picking be automatic #774
Description
In my previous PR #705 , I introduced platform auto-detect by a set of MSBuild statements, but it's abandoned due to its size. This makes the standalone variant development very easy. By keeping functional test projects' Configuration property same as Steamwork.NET, contributors can directly reference standalone v2's project file, and start their contributing, without wasting time for configuration map setup. This also useful to people who don't willing to use <PackageReference> for some reasons, because it makes building Steamworks.NET standalone is simple as above: add and reference Steamworks.NET.Standard.csproj, then copy steam binaries to their output/dlopen search path, it works.
Switching OS configuration to a MSBuild property have another benefits: If we want to make debugging easier like statement level breakpoints, we can't let OS configuration lays in <Configurations></Configurations>, or a lot of Windows-Debug | x64 and Linux-OSX-Debug | arm64 will appear.
Now #705 has been split into #745 and potential 2 upcoming PRs. One is confirmed, for NuGet packing workflow fix. The another is what we talk above. Due to platform picking changes, NuGet fix is dependent (or at least after) this. If we mostly agree with automatic picking, I will combine both PR into one, estimated 15- lines in csproj, 10- lines in BuildPackages.targets .