C#: Ensure native handles are freed after switch to .NET Core

Finalizers are longer guaranteed to be called on exit now that
we switched to .NET Core. This results in native instances leaking.

The only solution I can think of so far is to keep a list of all
instances alive to dispose when the AssemblyLoadContext.Unloading
event is raised.

modules/mono/csharp_script.cpp

@@ -1581,6 +1581,7 @@ void CSharpLanguage::tie_managed_to_unmanaged_with_pre_setup(GCHandleIntPtr p_gc
 	CSharpInstance *instance = CAST_CSHARP_INSTANCE(p_unmanaged->get_script_instance());
 
 	if (!instance) {
+		// Native bindings don't need post-setup
 		return;
 	}