package.json: add node-gyp as an optional peer dep

This way, the implicit dependency on node-gyp for local builds is explicit, and build tools like dream2nix don't need workarounds to add it :-)

You know what - I now see that node-gyp ships with npm by default (thanks for pointing that out) and it's just a matter of making sure it's in the path while building outside of npm. I was calling node-gyp directly and it was missing. Thanks again and sorry for the disturbance :+1: