LibWeb: Add special handling for WindowProxy in [Replaceable] setters

Setters for Window object should consider WindowProxy wrapper by:
- Verifying `this_value` is `WindowProxy` (not `HTML::Window`)
- Defining properties on the underlying Window object instead of on
  the WindowProxy itself.

Tests/LibWeb/Text/input/window-event-property-setter.html

@@ -0,0 +1,7 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        window.event = "hello";
+        println(window.event);
+    });
+</script>