repl: refactor code for readability

PR-URL: https://github.com/nodejs/node/pull/17919


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Lance Ball <lball@redhat.com>