Add theme preview (#20039)

This PR adds a theme preview tab to help get an at a glance overview of
the styles in a theme.

![CleanShot 2024-10-31 at 11 27
18@2x](https://github.com/user-attachments/assets/798e97cf-9f80-4994-b2fd-ac1dcd58e4d9)

You can open it using `debug: open theme preview`.

The next major theme preview PR will move this into it's own crate, as
it will grow substantially as we add content.

Next for theme preview:

- Update layout to two columns, with controls on the right for selecting
theme, layer/elevation-index, etc.
- Cover more UI elements in preview
- Display theme colors in a more helpful way
- Add syntax & markdown previews


Release Notes:

- Added a way to preview the current theme's styles with the `debug:
open theme preview` command.

crates/gpui/src/color.rs

@@ -1,7 +1,7 @@
 use anyhow::{bail, Context};
 use serde::de::{self, Deserialize, Deserializer, Visitor};
 use std::{
-    fmt,
+    fmt::{self, Display, Formatter},
     hash::{Hash, Hasher},
 };
 
@@ -279,6 +279,19 @@ impl Hash for Hsla {
     }
 }
 
+impl Display for Hsla {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "hsla({:.2}, {:.2}%, {:.2}%, {:.2})",
+            self.h * 360.,
+            self.s * 100.,
+            self.l * 100.,
+            self.a
+        )
+    }
+}
+
 /// Construct an [`Hsla`] object from plain values
 pub fn hsla(h: f32, s: f32, l: f32, a: f32) -> Hsla {
     Hsla {