LibWeb: Add HTMLSelectElement showPicker()

Adds the showPicker() function for select elements.
This doesn't do the check for "being rendered" which is in the spec.

Userland/Libraries/LibWeb/HTML/HTMLSelectElement.idl

@@ -37,5 +37,7 @@ interface HTMLSelectElement : HTMLElement {
     [FIXME] boolean reportValidity();
     [FIXME] undefined setCustomValidity(DOMString error);
 
+    undefined showPicker();
+
     readonly attribute NodeList labels;
 };