[12.x] Feature: Array partition without preserving keys

[liamduckett]

Hey, this PR allows using the existing Arr::partition method, without preserving keys.

I haven't added this to the EnumeratesValue trait (which Collection and LazyCollection make use of), due to it's additional complexity of operatorForWhere. This is not a problem (despite it's reliance on Arr::partition due to the new parameter being optional.

[taylorotwell]

You can just call values on the results.

[liamduckett]

@taylorotwell this feels very similar to #54916.

Calling array_values on the results feels a little verbose to me.

$array = ['John', 'Jane', 'Greg'];
$results = Arr::partition($array, fn (string $value) => str_contains($value, 'J'));

// Currently
$results = array_map(
    fn(array $result) => array_values($result),
    $results
);