Respect addAsFirstChild in addNodeUnderParent

[manuelmager]

Expected Behavior

When adding a new node to an existing treeData using addNodeUnderParent, I can get it to be the first element directly under root by passing parentKey: null, addAsFirstChild: true.

Actual Behavior

The flag addAsFirstChild is not respected, the new node is always appended as last child.

Steps to reproduce

    const result = addNodeUnderParent({
      treeData: [
        { key: 0, children: [] },
        { key: 6 },
      ],
      newNode: { key: 'new' },
      parentKey: null,
      getNodeKey: ({ node }) => node.key,
      addAsFirstChild: true,
    });

[coveralls]

Coverage decreased (-0.002%) to 75.577% when pulling 61db04d on manuelmager:fix/AddNodeUnderParent into cdc5fa3 on frontend-collective:master.

[wuweiweiwu]

LGTM!!

can you run prettier?

[manuelmager]

Should be pretty now. :)