update task-item.ts nodeview to update data-checked

[BrianHung]

#1566

Another alternative would be to remove the label wrapper element so the nodeview output is

<li data-checked="true" class="">
  <input type="checkbox">
  <div><p>A list item</p></div>
</li>

This would allow people to style the content of the todo-item with a sibling css selector:

ul[data-type=taskList] > li > input[type=checkbox] + div { 
  text-decoration: line-through;
}

[hanspagel]

Thanks for the fix @BrianHung! 🙌