Add more tools to conveniently work with hierarchies, especially in the presence of ghost nodes

## What problem does this solve or what need does it fill?

Following the introduction of https://github.com/bevyengine/bevy/pull/15341, some users have expressed concerns around difficulties migrating to account for ghost nodes in their hierarchies.

This is a great opportunity to add more tools for traversing hierarchies in the spirit of [`iter_descendants`](https://docs.rs/bevy/latest/bevy/hierarchy/trait.HierarchyQueryExt.html#tymethod.iter_descendants).

## What solution would you like?

There are two stages to this:

1. Add more methods to `HierarchyQueryExt` to capture other common tasks.
2. Add a ghost-node aware version of this trait, which lives in `bevy_ui` and takes a `Has` argument as well. These methods should be named `iter_node_descendants` and so on.

In theory these can be done in either order, or in a single PR. Your choice!

In terms of additional tasks to cover:

1. `root_parent`: the entity that is highest up the hierarchy tree, if any.
2. `leaf_children`: an iterator over the children that have no children of their own.
3. `siblings`: other entities that are in direct children of the same parent.
4. `parent`: a convenience method that looks for the parent (much more useful when working with ghost nodes).
5. `children`: as above, but for children.

## What alternative(s) have you considered?

One day (🥺), this will get rolled into #3742. It's nice to ship helpful APIs to users sooner though, and these methods will be easier to migrate since they have semantic meaning.