[8.x] Add collection sliding method

[JosephSilber]

This method was extracted from this old PR.

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Adds a sliding method to the collection, which creates chunks representing a "sliding window" view of the items in the collection:

Collection::times(5)->sliding(2); // [[1, 2], [2, 3], [3, 4], [4, 5]]
Collection::times(5)->sliding(3); // [[1, 2, 3], [2, 3, 4], [3, 4, 5]]

Collection::times(5)->sliding(3, step: 2); // [[1, 2, 3], [3, 4, 5]]

Examples:

1. Imagine you have a collection of nodes, which you want to link up into a linked list.

   The sliding method makes that really trivial:

   $nodes->sliding(2)->eachSpread(fn ($parent, $child) => $parent->next = $child);

   You can see similar usage in the original PR.

2. Or imagine you have a list of transactions, and want to fill in their running total:

   $transactions->sliding(2)->eachSpread(
       fn ($previous, $current) => $transaction->total = $previous->total + $current->amount
   );

[jesseschutt]

This would have come in quite handy in a situation where I had to compare a collection of Carbon instances and confirm each was within 15 minutes of the previous!

[michaeldyrynda]

That second example is something I had to implement in my last job, where we wanted to show time between consecutive records showing time offline between connected sessions. Being able to have a a chunk of that functionality built in to the framework would have saved headaches and a good few lines of code. If only I knew to search for a 'sliding window', I could've saved myself the headache.