LibWeb: Add the Document.documentElement API

Also change DOM::Document::document_element() to return an Element*
and not an HTML::HTMLHtmlElement since that's not the only kind of
documentElement we might encounter.

Libraries/LibWeb/Tests/Document/documentElement.js

@@ -0,0 +1,8 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("Basic functionality", () => {
+        expect(document.documentElement).not.toBe(null);
+        expect(document.documentElement.nodeName).toBe("html");
+    });
+});