tools: add update-npm script

I've had to update this one too many times, and much of it can be
automated, so let's do that!

The maintenance instructions have also been updated.

PR-URL: https://github.com/nodejs/node/pull/35822


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>