[10.x] Add Arr::mapWithKeys()

[ralphjsmit]

Personally I use the Collection mapWithKeys() method a lot. It does happen quite often still that I have just an array and need to return only an array. Converting to a collection, mapping with the keys and converting back to an array does not feel right if we have special Arr class for such array-only operations.

This PR adds the mapWithKeys() function to the Arr class.

public function getSomething(): array
{
    $data = [/** */];

    return Arr::mapWithKeys($data, function($x): array {
       // return [$y => $z]
    });
}

Thanks!