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

These get the next and previous sibling nodes, respectively.
We can already kind of do this by using the $child array, b

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

These get the next and previous sibling nodes, respectively.
We can already kind of do this by using the $child array, but that's
inconvenient when actually walking the tree by only using node
instances. Since the class is final, there is no BC break here.

Closes GH-15047.

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