refactor ui/borders example to use new children! macro (#18962)

# Objective

Refactor
[`examples/ui/borders.rs`](https://github.com/bevyengine/bevy/blob/7f0490655c3ede945e6babc1f1a1f06b13404aa8/examples/ui/borders.rs)
to use the new spawning/hierarchy APIs in 0.16.

## Solution

This refactor reduces the number of `.spawn` calls from about 16 to 2,
using one spawn for each major feature:

* camera2d
* ui layout

The `Children::spawn` relationship API is used to take advantage of
`SpawnIter` for the borders examples in each block.

Each block of examples now returns a Bundle into its respective
variable, which is then used in combination with the new `label` widget
which makes use of the new `impl Bundle` return capability. This allows
the ui layout to use a single `.spawn` with the `children!` macro.

The blocks of examples are still in separate variables because it felt
like a useful way to organize it still, even without needing to spawn at
those locations.

Functionality of the demo hasn't changed, this is just an API/code
update.

## Showcase

![screenshot-2025-04-27-at-18 05
19@2x](https://github.com/user-attachments/assets/5f10b28b-b0f1-4a55-af9f-2e7b05b7b4bf)

<details>
<summary>Before screenshot</summary>

![screenshot-2025-04-27-at-18 17
12@2x](https://github.com/user-attachments/assets/ae159d96-ba4d-4429-b934-7779470c480a)

</details>

---------

Co-authored-by: François Mockers <[email protected]>

Author: ChristopherBiscardi