LibWeb: Add {,de}serialization steps for DOMRect

Author: https://github.com/kennethmyhra
Commit: 94c6389fc0
Pull-request: https://github.com/SerenityOS/serenity/pull/23538
Reviewed-by: https://github.com/ADKaster

Tests/LibWeb/Text/input/HTML/StructuredClone-serializable-objects.html

@@ -39,6 +39,10 @@
         println(`instanceOf DOMRectReadOnly: ${domRectReadOnly instanceof DOMRectReadOnly}`);
         println(`DOMRectReadOnly: ${JSON.stringify(domRectReadOnly)}`);
 
+        let domRect = structuredClone(new DOMRect(10, 20, 30, 40));
+        println(`instanceOf DOMRect: ${domRect instanceof DOMRect}`);
+        println(`DOMRect: ${JSON.stringify(domRect)}`);
+
         done();
     });
 </script>