LibWeb: Add stub for ValidityState

This fixes https://html5test.com/ as previously an exception was being
thrown after trying to access this attribute which would then result in
a popup about the test failing (and none of the test results being
shown).

Userland/Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -174,6 +174,8 @@ public:
     virtual void form_associated_element_was_removed(DOM::Node*) override;
     virtual void form_associated_element_attribute_changed(FlyString const&, Optional<String> const&) override;
 
+    JS::NonnullGCPtr<ValidityState const> validity() const;
+
     // ^HTMLElement
     // https://html.spec.whatwg.org/multipage/forms.html#category-label
     virtual bool is_labelable() const override { return type_state() != TypeAttributeState::Hidden; }