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

[tznind]

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'

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Please upvote 👍 this pull request if you are interested in it.

[CLAassistant]

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[thoemmi]

k.item.Data must not be string but can be any type of object. You have to respect a potential converter. See how the data is rendered e.g. in SelectionPrompt:

spectre.console/src/Spectre.Console/Prompts/SelectionPrompt.cs

Line 166 in 8fa53f8

var text = (Converter ?? TypeConverterHelper.ConvertToString)?.Invoke(item.Node.Data) ?? item.Node.Data.ToString() ?? "?";

When you want to jump to an item with the specific starting character, you have to look for the string that is actually displayed.

[tznind]

Thanks for pointing this out, I had missed that. I have added the converter arg to ListPromptState<T>.Update method so it can check the actual string contents. I added it as a default parameter to the method but if you would rather I create a new overload instead then I can update to that approach.

I had to add the converter property to IListPromptStrategy<T> since that is what ListPrompt.cs has access to. And ListPrompt is the one calling state.Update.