Add callback logic for the showOnPreview() method

[guiribmedeiros]

Hello,

I saw that there is no callback logic for the showOnPreview() method:

     /**
     * Show the field in the modal preview.
     *
     * @return $this
     */
    public function showOnPreview()
    {
        $this->showOnPreview = true;

        return $this;
    }

And, to keep it consistent with all other similar methods, my suggestion is to add the callback logic to the showOnPreview() method, see the showOnDetail as an example:

     /**
     * Specify that the element should be hidden from the detail view.
     *
     * @param  (callable():bool)|bool  $callback
     * @return $this
     */
    public function showOnDetail($callback = true)
    {
        $this->showOnDetail = $callback;

        return $this;
    }

This would be nice, I missed that feature and hope other devs may miss that too, but the major reason is to keep it consistent ✌️