TreeSliver & TreeView alternate focus traversal options

[Piinks]

Follow up to #147171 and flutter/packages#6592

These PRs originally contained a tree traversal order concept for those that wanted to realize breadth first traversal order (visiting all nodes at a given depth before moving on to the next depth), in addition to the default (which is depth first).

At the time we landed those PRs, we did not know of anyone that wanted this feature, so we removed it for the time being until we could gather more information about the need and use cases of this feature.