repl: convert var to let and const

Refs: https://github.com/nodejs/node/pull/29535

This PR replaces the instances of var with let/const.

PR-URL: https://github.com/nodejs/node/pull/29575


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>