[9.x] Add soleValue method to Collections

[ash-jc-allen]

Hey! This PR proposes a small soleValue method that can be added to Collections and LazyCollections.

The new method tries to mimic the soleValue that's available in the query builder (added in this PR: #41368).

The idea behind using this is that it gives you more confidence that:

1. Exactly one items in the collection
2. The key definitely exists in the item that we are accessing

As a basic example, before this method, I might have written something similar to this:

$item = $collection->where('name', 'foo')->sole();

if (!array_key_exists('bar', $item)) { 
    throw new Exception('The "bar" field does not exist in the item');
}

return $item->bar;

But with the new method, I could write it like this:

return $collection->where('name', 'foo')->soleValue('bar');

If the item doesn't exist, an Illuminate\Support\InvalidKeyException exception will be thrown.

If you think this might be something worth merging and you need any changes making to it, please let me know and I'll sort it ASAP 😄

[driesvints]

@ash-jc-allen can you rebase with 9.x? We fixed the failing build there.

[ash-jc-allen]

@driesvints Sweet! I've done that now 🙂

[taylorotwell]

Think I'm going to hold off on this one for now. You can macro it in.

[ash-jc-allen]

That's no problem at all, I'll use a macro 🙂