Use NODE_ENV from environment instead of overriding it

webpack/index.js

@@ -0,0 +1,13 @@
+require('dotenv').config();
+
+const { NODE_ENV } = process.env;
+
+switch(NODE_ENV) {
+case 'development':
+case 'production':
+case 'test':
+  module.exports = require(`./${NODE_ENV}`); break;
+default:
+  console.error('ERROR: NODE_ENV must be set to either `development`, `test`, or `production`.');
+  process.exit(1);
+}