Add stack index to Node (#9853)

# Objective

If we add the stack index to `Node` then we don't need to walk the
`UiStack` repeatedly during extraction.

## Solution

Add a field `stack_index`  to `Node`.
Update it in `ui_stack_system`.
Iterate queries directly in the UI's extraction systems.

### Benchmarks 
```
cargo run --profile stress-test --features trace_tracy --example many_buttons -- --no-text --no-borders
```

frames (yellow this PR, red main):

<img width="447" alt="frames-per-second"
src="https://github.com/bevyengine/bevy/assets/27962798/385c0ccf-c257-42a2-b736-117542d56eff">

`ui_stack_system`:
<img width="585" alt="ui-stack-system"
src="https://github.com/bevyengine/bevy/assets/27962798/2916cc44-2887-4c3b-a144-13250d84f7d5">

extract schedule:
<img width="469" alt="extract-schedule"
src="https://github.com/bevyengine/bevy/assets/27962798/858d4ab4-d99f-48e8-b153-1c92f51e0743">

---

## Changelog

* Added the field `stack_index` to `Node`.
* `ui_stack_system` updates `Node::stack_index` after a new `UiStack` is
generated.
* The UI's extraction functions iterate a query directly rather than
walking the `UiStack` and doing lookups.