Add support for local npm prefix

[dnicolson]

This pull request adds support for the npm_config_local_prefix environment variable as well as the npm_config_prefix one documented here.

This enables submodules to work with a Go binary:

const path = require('path');
const { spawn } = require('child_process');

spawn(`${path.dirname(require.main.filename)}/node_modules/.bin/binary`);

[andreynering]

Thanks @dnicolson!