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/HTMLSelectElement.h

@@ -58,6 +58,8 @@ public:
 
     Vector<GC::Root<HTMLOptionElement>> list_of_options() const;
 
+    bool will_validate();
+
     // ^EventTarget
     // https://html.spec.whatwg.org/multipage/interaction.html#the-tabindex-attribute:the-select-element
     // https://html.spec.whatwg.org/multipage/interaction.html#focusable-area