Dispatch event on sortable update

[chrispage1]

When sorting is performed, there is currently no way to clear the cache (just one use case).

This contribution fixes this by dispatching an event.

The event dispatch allows us to listen for an event and perform whatever we need to do after the sort.

I have also added a convenience method called isFor which allows you to check directly against another string or model instance.

An example usage of this would be:

use Spatie\EloquentSortable\EloquentModelSortedEvent as SortEvent;

class SortingListener
{

    public function handle(SortEvent $event): void {
        if ($event->isFor(MyClass::class)) {
            // ToDo: flush our cache
        }
    }
}

Thanks!

[chrispage1]

Closed because I noticed I'd merged main 🤦

Sorry!