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/workspace/src/workspace.rs

@@ -9,6 +9,7 @@ pub mod searchable;
 pub mod shared_screen;
 mod status_bar;
 pub mod tasks;
+mod theme_preview;
 mod toolbar;
 mod workspace_settings;
 
@@ -323,6 +324,7 @@ pub fn init_settings(cx: &mut AppContext) {
 pub fn init(app_state: Arc<AppState>, cx: &mut AppContext) {
     init_settings(cx);
     notifications::init(cx);
+    theme_preview::init(cx);
 
     cx.on_action(Workspace::close_global);
     cx.on_action(reload);