copyWithIn() with Array-like

Currently, following behavior.

```js
import { copyWithin } from "@immutable-array/copyWithin";
const arrayLike = { length: 5, 3: 1 };
let actual = copyWithin(arrayLike, 0, 3);
assert.deepStrictEqual(actual, [1, , , 1,]);
```

Native `Array#copyWithIn()` work following:

```js
[].copyWithin.call({length: 5, 3: 1}, 0, 3);
// {0: 1, 3: 1, length: 5}
```

It return just array-like object.

Is this no problem?

Start at the @immutable-array/copyWithin entry point and compare its array-like behavior with Array#copyWithin.call using the supplied example. Confirm whether the intended result is the original array-like object or the returned array, then check for a relevant regression test and define the expected result before changing anything.

索引モデルが issue の本文から書いたものです。