Some improvements

[AigioL]

• Use Central Package Management (CPM) unified management of nuget packages
• Use .Net5+ new OperatingSystem.IsWindows/IsMacOS ... API
• Fix the type Avalonia.Win32.dll not being found when trimmed Avalonia.Win32.WindowImpl on non-Windows platforms.
  • https://github.com/BeyondDimension/SteamTools/issues/1449

Unhandled exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.TypeLoadException: Could not load type 'Avalonia.Win32.WindowImpl' from assembly 'Avalonia.Win32, Version=255.255.255.255, Culture=neutral, PublicKeyToken=c8d484a7012f9a8b'.
   at FluentAvalonia.UI.Controls.CoreWindow..ctor()

• Add SupportedOSPlatformAttribute / SupportedOSPlatformGuardAttribute
• Add Microsoft.SourceLink.GitHub