Add findBySlugOr() method with callback functionality

[hendurhance]

PR Description:

This pull request adds the findBySlugOr() method to the HasSlug trait, providing a new feature that allows finding a model by its slug or executing a callback if the slug is not found.

Changes Made:

• Added the findBySlugOr() method to the HasSlug trait.
• Implemented the callback functionality to execute custom logic when the slug is not found.
• Added a static method missingCallback exception to be thrown when a callback is missing

$slug = 'some-slug';

// Try to find a model with the given slug 'some-slug'.
// If not found, execute the callback and return its result.
$foundModel = Model::findBySlugOr($slug, ['*'], function ($slug) {
    // Your custom logic here, for example, create a new model with the given slug.
    $newModel = new Model(['slug' => $slug, 'name' => 'New Model']);
    $newModel->save();
    return $newModel;
});

Testing

• Included tests to verify that the callback is executed correctly when the slug is not found.

[freekmurze]

Thanks for your work on this.
To keep the package simple, I'm going to pass on this for now.