LibWeb: Implement document.scrollingElement

This returns a reference to the element that scrolls the document. In
standards mode it is equivalent to `document.documentElement`.

Tests/LibWeb/Text/input/document-scrollingElement-quirks-mode.html

@@ -0,0 +1,8 @@
+<!-- quirks mode -->
+<script src="include.js"></script>
+<script type="">
+    test(() => {        
+        println(`document.compatMode: ${document.compatMode}`);
+        println(`document.scrollingElement is body element: ${document.scrollingElement === document.body}`);
+    });
+</script>