feat: add `toChildModel` method

[jaulz]

It would be great if there was a toChildModel method that I can easily use to convert parent models to child models. Please let me know if you like the idea so I can create corresponding tests.

[tonysm]

So this would be used if the class you're instantiating is the parent class, right? Something like:

$obj = new User(['type' => 'admin', ....]); // Instance of User class
$obj->toChildModel(); // Creates an instance of the Admin class, passing along the attributes

I think I like that.

[jaulz]

Yep, exactly. I can add tests if you are okay with it 😊