title: Cypress and cy undefined in commands.js, but everything works
body: <p>When typing a custom command into <code>commands.js</code>, WebStorm's linter says that both <code>Cypress</code> and <code>cy</code> are undefined and does not provide any IntelliSense.
  Both are perfectly defined in any <code>integration</code> files.</p>
  <p>commands.js</p>
  <pre class="lang-js prettyprint-override"><code>Cypress.Commands.add('command', () =&gt; {
    cy.get('div');
  });
  
  // ESLint: 'Cypress' is not defined.(no-undef)
  // ESLint: 'cy' is not defined.(no-undef)
  </code></pre>
  <p>index.js</p>
  <pre class="lang-js prettyprint-override"><code>import './commands.js'
  </code></pre>
  <p>This error does not appear in VSCode. Instead both <code>Cypress</code> and <code>cy</code> are defined as <code>any</code>.</p>
  <p>Regardless of this error, however, the tests work fine.</p>
  <p>What can I do to have the linter recognize Cypress in its own files?</p>