[5.4] Adds ability to eager load counts via property

[regularlabs]

This PR adds the ability to eager load counts via the protected property $withCount in the same way you can eager load relations with $with.
Like:

class FooBar extends \Illuminate\Database\Eloquent\Model
{
    protected $with      = ['foo', 'bar'];
    protected $withCount = ['baz', 'qux'];

    ...
}

[tillkruss]

Tests are failing.

[regularlabs]

To pass the checks I have also added an early return in the withCount method, so that it does not do anything to the query builder when the passed relations is empty.

[sausin]

@regularlabs Thanks for this! 👍

I was not able to use nested relationships the same it works with ->with('first.second'). Is this a possibility? The name convention system could go follow the same setup and be something like first_second_count

The error that I get with the current implementation for the above example is:

BadMethodCallException with message 'Call to undefined method Illuminate\Database\Query\Builder::first.second()'

[decadence]

@sausin It just uses withCount method which doesn't support nested relations.

[sausin]

@decadence Thanks for clearing it up! Should've checked the changes properly