Add ui::ComponentPreview (#20319)

The `ComponentPreview` trait enables rendering storybook-like previews
of components inside of Zed.


![CleanShot 2024-11-06 at 16 32
25@2x](https://github.com/user-attachments/assets/6894663f-1bbc-4a40-b420-33882e9e239a)


This initial version will work for any component that doesn't return a
view.

Example impl:

```rust
impl ComponentPreview for Checkbox {
    fn description() -> impl Into<Option<&'static str>> {
        "A checkbox lets people choose between opposing..."
    }

    fn examples() -> Vec<ComponentExampleGroup<Self>> {
        vec![
            example_group(
                "Default",
                vec![
                    single_example(
                        "Unselected",
                        Checkbox::new("checkbox_unselected", Selection::Unselected),
                    ),
                    // ... more examples
                ],
            ),
            // ... more examples
        ]
    }
}
```

Example usage:

```rust
fn render_components_page(&self, cx: &ViewContext<Self>) -> impl IntoElement {
        v_flex()
            .gap_2()
            .child(Checkbox::render_component_previews(cx))
            .child(Icon::render_component_previews(cx))
    }
}
```

Release Notes:

- N/A

crates/ui/src/prelude.rs

@@ -9,6 +9,7 @@ pub use gpui::{
 
 pub use crate::styles::{rems_from_px, vh, vw, PlatformStyle, StyledTypography, TextSize};
 pub use crate::traits::clickable::*;
+pub use crate::traits::component_preview::*;
 pub use crate::traits::disableable::*;
 pub use crate::traits::fixed::*;
 pub use crate::traits::selectable::*;