code: function getPreviousItem(opts: Options, value: Value, block?: Block): ?Block {
      const { document, startBlock } = value;
      block = block || startBlock;
  
      const currentItem = getCurrentItem(opts, value, block);
      if (!currentItem) {
          return null;
      }
  
      const previousSibling = document.getPreviousSibling(currentItem.key);
  
      if (!previousSibling) {
          return null;
      } else if (previousSibling.type === opts.typeItem) {
          return previousSibling;
      }
      return null;
  }
docs: Return the previous item, from current selection or from a node.