[12.x] Feature: Collection chunk without preserving keys

[liamduckett]

Hey, this PR allows using the existing Collection::chunk and LazyCollection::chunk methods, without preserving keys.

I haven't added the new parameter to the Enumerable interface (despite the chunk method being declared in it) for backwards compatibility. Although Collection and LazyCollection are the frameworks only implementations of Enumerable, end user code may have additional implementations.

The new parameter does not break existing adherence to the interface, as it is optional (due to having a default value).

[liamduckett]

My first pass looked more like:

return new static(function () use ($size, $preserveKeys) {
    $iterator = $this->getIterator();

    while ($iterator->valid()) {
        $chunk = [];

        while (true) {
            if ($preserveKeys === false) {
                $chunk[] = $iterator->current();
            } else {
                $chunk[$iterator->key()] = $iterator->current();
            }

            if (count($chunk) < $size) {
                $iterator->next();

                if (! $iterator->valid()) {
                    break;
                }
            } else {
                break;
            }
        }

        yield new static($chunk);

        $iterator->next();
    }
});

I opted for this (match) approach due to LazyCollections perfomance oriented nature (appreciate this is at the cost of readability).