Make TUI indifferent to SHIFT when requiring letter keypresses

[mpilgrem]

On Windows 11, at least, when GHCup's TUI says q:Quit it literally means lowercase q only; Q does nothing - and the same for other letter keys that have functionality. (I discovered this by, accidentally, having CAPSLOCK set and being bemused why the TUI would not exit.)

I think the TUI can, and should, be indifferent to SHIFT when interpreting letter key presses.

EDIT: I see this would, however, mess significantly with how things have been set up, namely: 'one key combination <--> one action'. Perhaps the better approach is via documentation. At the moment, a user has to read, and interpret, https://github.com/haskell/ghcup-hs/blob/master/data/config.yaml to understand that, for example, q means a specific 'key combination'.