readline: improve Unicode handling

Prevents moving left or right from placing the cursor in between code
units comprising a code point.

PR-URL: https://github.com/nodejs/node/pull/25723
Fixes: https://github.com/nodejs/node/issues/25693


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>