repl: improve preview length calculation

The preview had an off by one error in case colors where deactivated
and did not take fullwidth unicode characters into account when
displaying the preview.

PR-URL: https://github.com/nodejs/node/pull/31112


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>