mono: add IsInstanceValid(), move WeakRef()

modules/mono/glue/cs_files/ObjectExtensions.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace Godot
+{
+    public partial class Object
+    {
+        public static bool IsInstanceValid(Object instance)
+        {
+            return instance != null && instance.NativeInstance != IntPtr.Zero;
+        }
+
+        public static WeakRef WeakRef(Object obj)
+        {
+            return NativeCalls.godot_icall_Godot_weakref(Object.GetPtr(obj));
+        }
+    }
+}