Update Model.php for nested relations

[Geovanek]

Updated model.php so that the relationSearch() method works with polymorphic relationships, and for a nested relationship.

For nested relations, in the relationSearch(), use:

public function relationSearch(): array
{
    return [
        'kitchen' => [ // relationship on dishes model
            'name', // column enabled to search
        ],
        'post' => [ //relationship on dishes model
            'category' => [ // relationship on nested model
                'name', // column enabled to search on nested model
            ],
        ],
        // ... others
    ];
}