feat: add fixed mode and more generic BaseAutocompleteItem

[JohnCampionJr]

This is my attempt to improve the BaseAutocomplete and BaseAutocompleteItem

I added:

• the allowCustom from #100
• ability to use fixed for the floating dropdown (I needed this in modals)
• made the BaseAutocompleteItem more generic and consistent with the BaseListbox

You should have edit permissions on this fork; open to ideas / suggestions.

I originally tried portals, but they seem to be very unreliable for multiple portalled elements and they also added styling challenges. I was able to fix my problems that started this with fixed positioning rather than portalled.

[JohnCampionJr]

Floating Headless really doesn't like two portalled items on the same page. Trying to see if there is a fix