Implement text and select block steps

[lox]

This provides an implementation of https://buildkite.com/docs/pipelines/block-step, using the UI components from https://github.com/manifoldco/promptui.

[lox]

@toolmantim the code is a mess, but I'd love feedback on the interaction design and things like spacing, etc.

[toolmantim]

This is awesome! There’s not a lot to say, it looks pretty good.

Perhaps the only two things I can see are the ✔️ character seems odd for a block, and that you could probably do without the final [y/n] — perhaps just unblock as soon as they fill out the last field? (Unless it’s a field-less block step, in which case you’ll probably still need it)

[lox]

I'm going to merge this for now!