[Fix] Properly replace koa-fix-trailing-slashes with inhouse script

Created by: daikei

This follows up #9869, with an awaited next(), fixing the script.

from #9689 (closed) I'm looking to remove another 5kb package, replacing it with an in-house script. The commit i'm proposing translates the entire job of koa-remove-trailing-slashes into a 206 byte script that gets the job done well.

(just to show the code does work)