use readline for prompt() via magic

[wbourne0]

The old logic for prompt was pretty bad and readline has all of the logic we need + more so I figured we should just use it instead of our own logic.

This does come with a contingency though - this works because stream.Readable, events.EventEmitter, and readline.Interface synchronously handle data pushed to streams. If that were to change (e.g. debouncing event dispatches) then this would fail.

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