Add ui::ContentGroup (#20666)

TL;DR our version of [HIG's
Box](https://developer.apple.com/design/human-interface-guidelines/boxes)

We can't use the name `Box` (because rust) or `ContentBox` (because
taffy/styles/css).

---

This PR introduces the `ContentGroup` component, a flexible container
inspired by HIG's `Box` component. It's designed to hold and organize
various UI elements with options to toggle borders and background fills.

**Example usage**:

```rust
ContentGroup::new()
    .flex_1()
    .items_center()
    .justify_center()
    .h_48()
    .child(Label::new("Flexible ContentBox"))
```

Here are some configurations:

- Default: Includes both border and fill.
- Borderless: No border for a clean look.
- Unfilled: No background fill for a transparent appearance.

**Preview**:

![CleanShot 2024-11-14 at 07 05
15@2x](https://github.com/user-attachments/assets/c838371e-e24f-46f0-94b4-43c078e8f14e)

---

_This PR was written by a large language model with input from the
author._

Release Notes:

- N/A

crates/ui/src/components.rs

@@ -1,6 +1,7 @@
 mod avatar;
 mod button;
 mod checkbox;
+mod content_group;
 mod context_menu;
 mod disclosure;
 mod divider;
@@ -36,6 +37,7 @@ mod stories;
 pub use avatar::*;
 pub use button::*;
 pub use checkbox::*;
+pub use content_group::*;
 pub use context_menu::*;
 pub use disclosure::*;
 pub use divider::*;