Added new chunk() collection method

[techmahedy]

Split a collection into smaller chunks of a specified size.

Example usages:

// Process large dataset in batches
$users = User::all(); // 1000 users
$batches = $users->chunk(100);

foreach ($batches as $batch) {
    // Process 100 users at a time
}

// Create pages for display
$products = Product::all();
$pages = $products->chunk(20); // 20 items per page