LibWeb: Implement :enabled and :disabled pseudo classes to spec

Previously we only considered an element disabled if it was an <input>
element with the disabled attribute, but there's way more elements that
apply with more nuanced disabled/enabled rules.

Userland/Libraries/LibWeb/DOM/Element.h

@@ -142,6 +142,8 @@ public:
     void clear_pseudo_element_nodes(Badge<Layout::TreeBuilder>);
     void serialize_pseudo_elements_as_json(JsonArraySerializer<StringBuilder>& children_array) const;
 
+    bool is_actually_disabled() const;
+
 protected:
     Element(Document&, DOM::QualifiedName);
     virtual void initialize(JS::Realm&) override;