Add support for compiling and running engine without C# scripting

(configurable via `EngineConfiguration.UseCSharp` in Flax.Build)

Source/Engine/Content/Asset.cpp

@@ -11,7 +11,9 @@
 #include "Engine/Profiler/ProfilerCPU.h"
 #include "Engine/Threading/MainThreadTask.h"
 #include "Engine/Threading/ConcurrentTaskQueue.h"
+#if USE_MONO
 #include <ThirdParty/mono-2.0/mono/metadata/mono-gc.h>
+#endif
 
 AssetReferenceBase::~AssetReferenceBase()
 {
@@ -202,7 +204,9 @@ void Asset::OnManagedInstanceDeleted()
     // Cleanup
     if (_gcHandle)
     {
+#if USE_MONO
         mono_gchandle_free(_gcHandle);
+#endif
         _gcHandle = 0;
     }