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/HTML/HTMLFieldSetElement.h

@@ -26,6 +26,8 @@ public:
         return fieldset;
     }
 
+    bool is_disabled() const;
+
     // ^FormAssociatedElement
     // https://html.spec.whatwg.org/multipage/forms.html#category-listed
     virtual bool is_listed() const override { return true; }