repl: use _REPL_ in user-facing text

User-facing error messages should use _REPL_ for the general REPL and
not _repl_ which should be reserved for referring to the module itself,
etc. _REPL_ is an acronym, so it should be capitalized, as it generally
is in documentation in other technologies, and is usually in our own
documentation. This fixes a few inconsistent usages in lib/repl.js.

PR-URL: https://github.com/nodejs/node/pull/34643


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>