LibWeb: Implement the form-control willValidate property

This change — part of the HTML constraint-validation API (aka
“client-side form validation”) — implements the willValidate IDL/DOM
attribute/property for all form controls that support it.

Libraries/LibWeb/HTML/HTMLObjectElement.h

@@ -47,6 +47,8 @@ public:
 
     virtual void visit_edges(Cell::Visitor&) override;
 
+    static bool will_validate();
+
 private:
     HTMLObjectElement(DOM::Document&, DOM::QualifiedName);