Implement RecursiveIterator

PHP has native interfaces and classes for Iterators (Generator implements Iterator). Iterators can be traversed with foreach and composed easily, for example by filtering through CallbackFilterIterator.

In particular, it has the RecursiveIterator interface, that adds two more methods to implement: hasChildren() and getChildren().
This allows it to be traversed with a RecursiveIteratorIterator, which has countless options for emitting all nodes or just leaves, using level order, etc. ParentIterator can be used to visit just nodes with child nodes, RecursiveLimitIterator to limit the nodes traversed, AppendIterator to concatenate Iterators, RecursiveTreeIterator can be used to draw an ASCII tree of an AST and RecursiveCallbackFilterIterator to filter nodes (including skipping children). Really a lot of awesome stuff in there.

The Parser already uses Generators for traversal, which is great, but it only has limited options for filtering and skipping certain children through a callback: https://sourcegraph.com/github.com/Microsoft/tolerant-php-parser/-/blob/src/Node.php#L158-159
I think providing class-based implementations for Iterators would give greater flexibility.

Start with the traversal code in src/Node.php around the Generator implementation linked in the issue, then review how the parser represents child nodes. Define what class-based RecursiveIterator support should cover, including hasChildren(), getChildren(), filtering, and skipping children; done means traversal can be composed through the PHP iterator APIs described in the issue.