LibWeb: Refactor "editable" and "editing host" concepts

The DOM spec defines what it means for an element to be an "editing
host", and the Editing spec does the same for the "editable" concept.
Replace our `Node::is_editable()` implementation with these
spec-compliant algorithms.

An editing host is an element that has the properties to make its
contents effectively editable. Editable elements are descendants of an
editing host. Concepts like the inheritable contenteditable attribute
are propagated through the editable algorithm.

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -411,7 +411,7 @@ static bool matches_read_write_pseudo_class(DOM::Element const& element)
         return true;
     }
     // - elements that are editing hosts or editable and are neither input elements nor textarea elements
-    return element.is_editable();
+    return element.is_editable_or_editing_host();
 }
 
 // https://www.w3.org/TR/selectors-4/#open-state