readline: replace _questionCancel with a symbol

This commit avoids exposing a new underscored property on
readline Interface instances.

PR-URL: https://github.com/nodejs/node/pull/37094


Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>