title: Extend @types/node with custom project environment variable type decelerations
body: <p>I have a project that uses <a href="https://npmjs.com/package/dotenv" rel="nofollow noreferrer"><strong><code>dotenv</code></strong></a> for environment variables, and I want to add custom project-specific typings for the variables the project uses, for autocompletion, etc.</p>
  
  <p>How can I do this without overriding all the stuff that <a href="https://npmjs.com/package/@types/node" rel="nofollow noreferrer"><strong><code>@types/node</code></strong></a> already does?</p>