[7.x] Implement HigherOrderWhenProxy for Collections

[lorisleiva]

Hi there, 👋

This PR allows us to use a higher-order proxy for the Collection::when method.

// With this PR, this:
$collection->when($condition, function ($collection) use ($item) {
    $collection->push($item);
});

// ... can be refactored as this:
$collection->when($condition)->push($item);

I have also added the __get() magic method so that we can even chain other higher-order proxy methods like so:

// This:
$collection->when($condition, function ($collection) {
    $collection->map->parseIntoSomething();
});

// ... can be refactored as this:
$collection->when($condition)->map->parseIntoSomething();

If this is something you'd like to include in the Laravel framework, I will add some tests and update the documentation accordingly.

Cheers. 🍀