Effectively only nodejs is packaged on Debian but not npm. You need to install it from here: https://github.com/nodesource/distributions

Is that an acceptable solution for the manual?

Considering there is no other way, I think so.

Good, I didn't want to rewrite webpack in a way that does not use npm

Other solution is to install nodejs from stretch-backports (as we need version 8.x LTS) and then manually install npm with

$ curl -L https://npmjs.org/install.sh | sudo sh

But I think installing it with the packages available on https://github.com/nodesource/distributions is
a better option.

In T1225#22798, @anlambert wrote:

$ curl -L https://npmjs.org/install.sh | sudo sh

over my dead body !

so, yeah, the +1 for the other option too