[8.x] Add missing sortByMany method to Arr helper

[johnylemon]

The need to sort array with multiple columns is the problem I faced so many times. Incuding today.
Arr and Str helpers are really powerfull and cover so many common use cases. But not this one.
This PR fills this gap, empowering helpers functionality.

Sorting may be done by array of strings:

$sorted = Arr::sortByMany($unsorted, [
    'name', 'age'
];

These props will be sorted in ascending order by default.

Order may be changed, using array instead of property name:

$sorted = Arr::sortByMany($unsorted, [
    ['name', false], // will be ordered by `name` descending  
    ['age', true] // then by age ascending, if there are items with same `name` value
];

Sometimes more complicated logic or calling object methods is needed, this may be achieved using anonymous functions:

$sorted = Arr::sortByMany($unsorted, [
    // order by childrens count asc
    function($a, $b) {
        return $a->childrens()->count() <=> $b->childrens()->count();
    },
    // then by grandchildrens by descending order if needed
    function($a, $b) {
        return $b->grandchildrens()->count() <=> $a->grandchildrens()->count();
    }
];

When using anonymous functions, order boolean parameter won't be respected.

[taylorotwell]

You can just pass an array to Arr::sort or Collection::sortBy now. Thanks.

[taylorotwell]