Add tidyNode::getNextSibling() and tidyNode::getPreviousSibling()

> Hi internals > > I opened a PR to add the getNextSibling() and getPreviousSibling() methods to the tidyNode class. > As the class is final, this has no BC break concerns. > > The reason for adding this is that it's inconvenient to walk the tree solely using the existing getParent() method and ->child property. > > Since it's a tree anyway, methods to obtain the previous or next sibling are natural en convenient. > > Just sending this mail to see if anyone objects to this. > PR link: https://github.com/php/php-src/pull/15047 > > Kind regards > Niels