Menu-Button, Listbox-button: clean up the configuration

[saiponnada]

I think one thing we might need to do is make it clearer that some of these options are only valid for the single select (i.e. role=menuitemradio) variant. Maybe we could move those into a sub mapping? Or any other ideas?

const defaultOptions = {
customElementMode: false,
expandedClass: "menu-button--expanded",
menuSelector: ".menu-button__menu",
buttonTextSelector: .btn__text,
singleSelect: {
buttonValueType: "text", // ["text", "icon", "both"],
menuItemIconSelector: ".icon",
menuItemButtonAriaLabelSelector: null,
}
};