readline: replace quadratic regex with linear one

Simplify regular expression in _wordLeft and _deleteWordLeft readline
methods.

PR-URL: https://github.com/nodejs/node/pull/26778


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>