fix: allow confirm to only accept 'y' or 'n' as input

[bryanoltman]

Description

We're seeing users accidentally enter input other than 'y' or 'n' to a confirm prompt that follows a long build step (see shorebirdtech/shorebird#1572). It would be a nice guardrail to only accept these inputs and re-display the prompt if we detect input that is not 'y' or 'n'. It would probably make sense to make this behavior optional and to not change the default behavior of confirm, maybe by adding an abortOnUnrecognizedInput argument to confirm that defaults to true.