`bevy_ui_debug` scrollbars inset fix (#22343)

# Objective

`ComputedNode`'s `content_inset` field was changed to allocate space for
the scrollbars between the content and padding areas. The UI debug
overlay expects the `content_insets` to include that space and as a
result wrongly draws the scrollbars inset into the content box by their
width.

## Solution

Use `ComputedNode`'s `horizontal_scrollbar` and `vertical_scrollbar`
methods to position the debug overlay's scroll bar rects instead.

## Testing

```
cargo run --example scroll --features="bevy_ui_debug"
```

On main you should see a gap between the the vertical scrollbar and the
right edge of the node.
With this PR, the gap should be gone.