Add stubs for ValTrait.next

valuescript_vm/src/native_frame_function.rs

@@ -70,6 +70,10 @@ impl ValTrait for NativeFrameFunction {
     type_error!("Cannot assign to subscript of native function")
   }
 
+  fn next(&mut self) -> LoadFunctionResult {
+    LoadFunctionResult::NotAFunction
+  }
+
   fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
     write!(f, "\x1b[36m[Function]\x1b[39m")
   }