title: In the latest mui LoadingButton is missing?
body: <p>As per the material UI documentation,
  <a href="https://next.material-ui.com/components/buttons/" rel="nofollow noreferrer">https://next.material-ui.com/components/buttons/</a></p>
  <p>you can
  <code>import LoadingButton from '@material-ui/lab/LoadingButton';</code>
  However, I don't see this folder anywhere in mui/lab and the import throws an error.</p>
  <p>I ran <code>yarn add @material-ui/lab</code> already.</p>
  <p>I also have <code>&quot;@material-ui/core&quot;</code> already.</p>
  <p>Do I need to install something for 'next mui' as I see the documentation url is also different:
  <a href="https://next.material-ui.com/" rel="nofollow noreferrer">https://next.material-ui.com/</a></p>
  <p>While most of components I have been using are here:
  <a href="https://material-ui.com/" rel="nofollow noreferrer">https://material-ui.com/</a></p>