support required fields

[jdmann]

I don't see a required prop on Listbox. This is needed for apps that rely on native form validation. Or will need to build our own logic for this.

[AnnaYuS]

It would be nice to have it for the Combobox as well. In Vue version there is a hidden input rendered if we add name prop but required doesn't work for a hidden input, so for now to trigger native validation we'll probably end up with adding an "invisible" input