Add ability to skip the slug generation by a condition

[masterix21]

Hi dears.

The PR adds the ability to skip the slug generation by a condition like so:

class Page extends Model
{
        HasSlug;

        public function getSlugOptions(): SlugOptions
        {
            return parent::getSlugOptions()
                ->skipGenerateWhen(fn () => $this->status === 'draft');
        }
}

[freekmurze]

Very nice, thank you!