readline: allow completer to rewrite existing input

Sometimes, it makes sense for a completer to change the existing
input, e.g. by adjusting the casing (imagine a completer that
corrects `Number.isNan` to `Number.IsNaN`, for example).

This commit allows that in the readline implemention.

PR-URL: https://github.com/nodejs/node/pull/39178


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>