Use cross-env For Env Variables

The cross-env package will help set and use enviroment variables across different platforms.

Example setting NODE_ENV

  "scripts": {
    "start": "cross-env NODE_ENV=production node index.js",
    "dev": "cross-env NODE_ENV=development nodemon index.js",
    "test": "cross-env NODE_ENV=test jest --verbose --runInBand"
  },