For WPF, Use Cross-Platform Implementation for BaseCommand.IsMainThread and BaseCommand.BeginInvokeOnMainThread (#965)

* Add Non-WPF Support to .NET Core 3.1

* Update comments

* Merge .NET Standard and WPF Functionality

Author: TheCodeTraveler

src/CommunityToolkit/Xamarin.CommunityToolkit/ObjectModel/Internals/BaseCommand.netstandard.cs renamed to src/CommunityToolkit/Xamarin.CommunityToolkit/ObjectModel/Internals/BaseCommand.netstandard.wpf.cs

@@ -13,7 +13,7 @@ public abstract partial class BaseCommand<TCanExecute>
 		static void BeginInvokeOnMainThread(Action action)
 		{
 			if (synchronizationContext != null && SynchronizationContext.Current != synchronizationContext)
-				synchronizationContext.Post((o) => action(), null);
+				synchronizationContext.Post(_ => action(), null);
 			else
 				action();
 		}