Add support for compiling and running engine without C# scripting

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

Source/Engine/Scripting/ManagedSerialization.h

@@ -12,6 +12,7 @@ class FLAXENGINE_API ManagedSerialization
 {
 public:
 
+#if USE_MONO
     /// <summary>
     /// Serializes managed object to JSON.
     /// </summary>
@@ -40,4 +41,5 @@ public:
     /// <param name="data">The input data.</param>
     /// <param name="object">The object to deserialize.</param>
     static void Deserialize(const StringAnsiView& data, MonoObject* object);
+#endif
 };