Add macroable to FactoryBuilder class

[jerguslejko]

This PR adds Macroable trait to the Illuminate\Database\Eloquent\FactoryBuilder class.

In my projects, I often use this bit of code inside of model factories:

'user_id' => function() {
    return User::first()->id ?? factory(User::class)->create()->id;
},

If macros were available, the above code could look more like this:

'user_id' => factory(User::class)->firstOrCreate(),

What do you think? 🌵