title: ERROR Invalid options in vue.config.js: "build" is not allowed. "dev" is not allowed
body: <p>I have an existing <code>vue-cli 2</code> app that I'm attempting to upgrade to <code>vue-cli-3</code>. After adding my unique dependencies,  I dropped <code>src/</code> right into the newly created <code>vue-cli-3</code> app and started up. <em>woot!</em></p>
  
  <p>How do I manage <code>[PROD|DEV|TEST].env.js</code> now that we use <code>vue.config.js</code>?</p>
  
  <p>I got the following error because my first attempt to create a <code>vue.config.js</code> was to simply rename <code>config/index.js</code> to be <code>/vue.config.js</code> and keep the existing <code>/config/[PROD|DEV|TEST].env.js</code> but I got the following error:</p>
  
  <pre><code>ERROR  Invalid options in vue.config.js: "build" is not allowed. "dev" 
  is not allowed error Command failed with exit code 1.
  </code></pre>
  
  <p>I don't understand how environments are now managed. </p>
  
  <p>Thanks for your time!</p>