Prototype is Losted

https://github.com/philipstanislaus/performant-array-to-tree/blob/efe07a8694e74a7dd114149961e2baa4db0c4397/src/arrayToTree.ts#L88

change

```js
lookup[itemId] = {
    ...item,
    [conf.childrenField]: lookup[itemId][conf.childrenField],
  };
```

to:

```js
lookup[itemId] = Object.assign(item, {
  [conf.childrenField]: lookup[itemId][conf.childrenField]
})
```