Allow letter keys (A-Z) to cycle indexes in Prompts

[tznind]

trafficstars

Adds the ability to press letter keys (A-Z) to cycle selection to next item beginning with that letter in Prompt.

I have made changes only to ListPromptState.cs as this is where Up and Down are being handled. Let me know if there is a better place for it or if this isn't a feature you want to incorporate.

I created a Discussion in #992 but since it was so easy to implement I thought I would just create a PR to demonstrate the idea.

Check up/down works then pressing 'M' repeatedly then 'L' then End followed by clycling 'O'

[CLAassistant]

All committers have signed the CLA.