LibWeb: Keep select button text in sync with the selected option

Previously, the select button's text was only refreshed inside the
two non-trivial branches of the selectedness setting algorithm.
Paths that left the select with exactly one selected option hit a
no-op branch and skipped the refresh.

Fix this by implementing the "clone selected option into select
button" algorithm and invoking it whenever the set of selected options
may have changed.

Libraries/LibWeb/HTML/HTMLOptionElement.cpp

@@ -54,7 +54,7 @@ void HTMLOptionElement::update_selection_label()
 {
     if (selected()) {
         if (auto* select_element = first_ancestor_of_type<HTMLSelectElement>()) {
-            select_element->update_inner_text_element({});
+            select_element->clone_selected_option_into_select_button();
         }
     }
 }