code: function increaseItemDepth(opts: Options, change: Change): Change {
      const previousItem = getPreviousItem(opts, change.value);
      const currentItem = getCurrentItem(opts, change.value);
  
      if (!previousItem) {
          return change;
      }
  
      if (!currentItem) {
          return change;
      }
  
      // Move the item in the sublist of previous item
      return moveAsSubItem(opts, change, currentItem, previousItem.key);
  }
docs: Increase the depth of the current item by putting it in a sub-list
  of previous item.
  For first items in a list, does nothing.