[8.x] Support shorter subscription syntax

[taylorotwell]

This PR allows you to return an array from the subscribe method (this was already supported) that has a shorter syntax for methods that exist on the subscriber itself:

<?php

namespace App\Listeners;

use App\Events\SomeEvent;

class SomeEventSubscriber
{
    /**
     * Handle an event.
     */
    public function handleSomeEvent($event)
    {
        info('Handling some event...');
    }

    /**
     * Register the listeners for the subscriber.
     *
     * @param  \Illuminate\Events\Dispatcher  $events
     * @return void
     */
    public function subscribe($events)
    {
        return [
            SomeEvent::class => [
                'handleSomeEvent',
            ],
        ];

        // Or...

        return [
            SomeEvent::class => 'handleSomeEvent',
        ];
    }
}

[binaryweavers]

I am not sure if I am on right path (correct me please if I am missing something). I was managing the other way around. Having listeners of an event be on itself. Did that for code management purpose so that by looking at an event I would know what are it's listeners 🤔
Doing something this in EventServiceProvider.php