lib: replace var with let and const in readline.js

PR-URL: https://github.com/nodejs/node/pull/30377


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>