Add task-list only to parent of task-list-item

There is an issue where the top-level list could be identified as a task-list when there is a sub-list that is a task list.

For example,

- Item one
  - [ ] subitem one

generates the following HTML, where the top list does not have the task-list class:

<ul>
  <li>Item one
    <ul class="task-list">
      <li class="task-list-item">
      <input type="checkbox" class="task-list-item-checkbox" disabled> subitem one</li>
    </ul>
  </li>
</ul>

However

- Item one
  - [ ] subitem one
- Item two
  - [ ] subitem two

does place a task-list class on the top level ul:

<ul class="task-list">
  <li>Item one
    <ul class="task-list">
      <li class="task-list-item">
      <input type="checkbox" class="task-list-item-checkbox" disabled> subitem one</li>
    </ul>
  </li>
  <li>Item two
    <ul class="task-list">
      <li class="task-list-item">
      <input type="checkbox" class="task-list-item-checkbox" disabled> subitem two</li>
    </ul>
  </li>
</ul>

This can confuse situations where identifying/styling the actual task-list is important.

This MR changes it so that only the direct parent ul of a task item is identified as a task-list, like the first example.