Add .list-item display utility

[neupauer]

This PR adds .list-item display utility.

It is the default value for the list item.

Adding a display: list-item to the element is useful in situations where you conditionally show/hide an item in the list, or toggling between flex and default behavior.

Also useful when creating fake lists.

E.g.: (This is just an example, I know it can be done like predicate ? '' : 'hidden')

<ul>
  <li>A</li>
  <li className={predicate ? 'list-item' : 'hidden'}>B</li>
  <li>C</li>
</ul>

<div>
  <div class="list-item">A</div>
  <div class="list-item">B</div>
</div>

Links:
MDN: display-listitem

[adamwathan]

Looks good, thank you!