Previously, we were using `smallestNotIn` for selecting new characters
for menu items with duplicate chatacters - this uses the 'Bounded'
instance for the type, which for Char meant the first character we would
always select was \NUL - making it look like the menu item had no
character, and making it impossible to actually select the menu item.
This introduces an AlphaChar newtype, which is a wrapper around Char
whose Bounded and Enum instances only use alphabetic characters (a-ZA-Z)
and uses that for menu characters instead.
Change-Id: If34ed9e9ce84f2bcb1cb87432cc6273f40b69f72
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3229
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
