Refactor CoreCLR runtime into explicit dotnet api instead of mocking mono api

Required by platforms that will use mono under the hood for .Net 7
New `USE_CSHARP` define for C# ability
Engine doesn't use `mono_*` apis directly but via MCore/MClass/MMethod/ apis

Source/Editor/Windows/AboutDialog.cs

@@ -125,6 +125,9 @@ namespace FlaxEditor.Windows
                 "Used third party software:",
                 "",
                 "Mono Project - www.mono-project.com",
+#if USE_NETCORE
+                ".NET - www.dotnet.microsoft.com",
+#endif
                 "FreeType Project - www.freetype.org",
                 "Assimp - www.assimp.sourceforge.net",
                 "DirectXMesh - Copyright (c) Microsoft Corporation. All rights reserved.",