Simplified `GhostNode`s implementation

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

`GhostNode`s force all code interacting with UI nodes to use the `UiChildren` and `UiRootNodes` system params, instead of regular queries that filter for `Children`/`ChildOf`. They add complexity, aren't performant, and are a constant and recurring source of bugs. In particular, incremental updates are significantly more difficult to implement because we have to consider `GhostNode` traversal.

## What solution would you like?

- Remove the `ghost_nodes` feature and all the feature gates.
- Remove the `UiChildren` and `UiRootNodes` systems params. Replace them with regular queries.
- Keep the `GhostNode` marker component. Have it require `Node` for the transitive requirements and propagation, but ignore `Node`s properties when the `GhostNode` marker is present.
- In `ui_layout_system` skip entities with `GhostNode` and replace them with their children during taffy layout updates.
- Update the `GhostNode` tests as needed.

## What alternative(s) have you considered?

Remove the `GhostNode` feature completely and reimplement the mechanism using BSN. I'm not sure what that would entail, but I assume it would be significantly more challenging to implement.

Start by locating the ghost_nodes feature gates, the UiChildren and UiRootNodes system params, the ui_layout_system, and the GhostNode tests. Trace how GhostNode traversal currently affects layout and propagation, then verify that regular Children/ChildOf queries, transitive Node requirements, and the updated tests cover the requested behavior.

Written by the indexing model from the issue text.