Update index.ts

[oztofer]

nested sortable compability

<ul data-controller="sortable">
  <li>1</li>
  <li>2</li>
  <li>3
    <ul class="nested-sortable">
      <li>3.1</li>
      <li>3.2</li>
      <li>3.3</li>
    </ul>
  </li>
  <li>4
    <ul class="nested-sortable">
      <li>4.1</li>
      <li>4.2</li>
      <li>4.3
        <ul class="nested-sortable">
          <li>4.3.1</li>
          <li>4.3.2</li>
          <li>4.3.3</li>
        </ul>
      </li>
    </ul>
  </li>
</ul>