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/workspace/src/theme_preview.rs

@@ -5,8 +5,8 @@ use theme::all_theme_colors;
 use ui::{
     element_cell, prelude::*, string_cell, utils::calculate_contrast_ratio, AudioStatus,
     Availability, Avatar, AvatarAudioStatusIndicator, AvatarAvailabilityIndicator, ButtonLike,
-    Checkbox, CheckboxWithLabel, DecoratedIcon, ElevationIndex, Facepile, IconDecoration,
-    Indicator, Table, TintColor, Tooltip,
+    Checkbox, CheckboxWithLabel, ContentGroup, DecoratedIcon, ElevationIndex, Facepile,
+    IconDecoration, Indicator, Table, TintColor, Tooltip,
 };
 
 use crate::{Item, Workspace};
@@ -510,6 +510,7 @@ impl ThemePreview {
             .overflow_scroll()
             .size_full()
             .gap_2()
+            .child(ContentGroup::render_component_previews(cx))
             .child(IconDecoration::render_component_previews(cx))
             .child(DecoratedIcon::render_component_previews(cx))
             .child(Checkbox::render_component_previews(cx))