[Proposal] Add isFirst() and isLast()

[ivanBell199]

Thank you for this great package. I have just started using it and it is very enjoyable. Unless I overlooked, it seems there are no methods to check if a node is first/last within its level. I think it would be useful for something like disabling "up" and "down" buttons when reordering is needed. In my project I simply adjusted methods up() and down():

    /**
     * Check if node is first among siblings.
     *
     * @return bool
     */
    public function isFirst()
    {
        $sibling = $this->prevSiblings()
            ->defaultOrder('desc')
            ->first();

        return $sibling ? false : true;
    }

    /**
     * Check if node is last among siblings.
     *
     * @return bool
     */
    public function isLast()
    {
        $sibling = $this->nextSiblings()
            ->defaultOrder()
            ->first();

            return $sibling ? false : true;
    }

Do you think it is worth being added to the package?