add a Searchable part and a AddTrigger part for the Select component.
The searchable part would be an input to search the options like this:
(Yes I know the Select already has type-ahead support, but I'm suggesting an actual search which filters the options)
The AddTrigger would be a button that appears at the end of the list and let the user create a new option if one doesn't exist yet. Similar to this: https://mantine.dev/combobox/?e=SelectCreatable