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"
},