Flatten to a depth

Implement flatten(arr, depth = 1), which behaves like arr.flat(depth). Don't call Array.prototype.flat or flatMap.

It returns a new array in which nested arrays are unwrapped up to depth levels. Unwrapping means the items of a nested array take the place of the array itself.

• depth defaults to 1. Infinity unwraps everything. 0 or a negative depth unwraps nothing and returns a shallow copy.
• The order of items is kept. An empty nested array that gets unwrapped leaves nothing behind.
• Holes in sparse arrays (indexes never assigned, as in [1, , 2]) are dropped in every array that gets unwrapped. Real undefined and null values are kept.
• Don't modify the input.

Constraints: up to 200 000 items in total, and arrays may be nested up to 100 000 levels deep.