Add role="combobox" to input element or allow this to be added as a prop

I'm using svelte-select as a combobox but there doesn't seem to be a way to mark this up correctly for accessibility purposes - see e.g. https://www.w3.org/WAI/ARIA/apg/patterns/combobox/

My suggestion is to add `role="combobox"` to the `input` element by default, or allow this to be added as a prop