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/DOM/Document.idl

@@ -9,6 +9,7 @@ interface Document : Node {
     readonly attribute DOMString compatMode;
     readonly attribute DocumentType? doctype;
 
+    readonly attribute Element? documentElement;
     readonly attribute HTMLElement? body;
 
 }