assert: align character indicators properly

This makes sure color codes are not taken into account in case
util.inspect's default value was changed.

PR-URL: https://github.com/nodejs/node/pull/31429


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>