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/ui/src/styles/elevation.rs

@@ -1,5 +1,8 @@
-use gpui::{hsla, point, px, BoxShadow};
+use std::fmt::{self, Display, Formatter};
+
+use gpui::{hsla, point, px, BoxShadow, Hsla, WindowContext};
 use smallvec::{smallvec, SmallVec};
+use theme::ActiveTheme;
 
 /// Today, elevation is primarily used to add shadows to elements, and set the correct background for elements like buttons.
 ///
@@ -15,6 +18,8 @@ pub enum ElevationIndex {
     Background,
     /// The primary surface – Contains panels, panes, containers, etc.
     Surface,
+    /// The same elevation as the primary surface, but used for the editable areas, like buffers
+    EditorSurface,
     /// A surface that is elevated above the primary surface. but below washes, models, and dragged elements.
     ElevatedSurface,
     /// A surface that is above all non-modal surfaces, and separates the app from focused intents, like dialogs, alerts, modals, etc.
@@ -25,11 +30,26 @@ pub enum ElevationIndex {
     DraggedElement,
 }
 
+impl Display for ElevationIndex {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            ElevationIndex::Background => write!(f, "Background"),
+            ElevationIndex::Surface => write!(f, "Surface"),
+            ElevationIndex::EditorSurface => write!(f, "Editor Surface"),
+            ElevationIndex::ElevatedSurface => write!(f, "Elevated Surface"),
+            ElevationIndex::Wash => write!(f, "Wash"),
+            ElevationIndex::ModalSurface => write!(f, "Modal Surface"),
+            ElevationIndex::DraggedElement => write!(f, "Dragged Element"),
+        }
+    }
+}
+
 impl ElevationIndex {
     /// Returns an appropriate shadow for the given elevation index.
     pub fn shadow(self) -> SmallVec<[BoxShadow; 2]> {
         match self {
             ElevationIndex::Surface => smallvec![],
+            ElevationIndex::EditorSurface => smallvec![],
 
             ElevationIndex::ElevatedSurface => smallvec![BoxShadow {
                 color: hsla(0., 0., 0., 0.12),
@@ -62,4 +82,17 @@ impl ElevationIndex {
             _ => smallvec![],
         }
     }
+
+    /// Returns the background color for the given elevation index.
+    pub fn bg(&self, cx: &WindowContext) -> Hsla {
+        match self {
+            ElevationIndex::Background => cx.theme().colors().background,
+            ElevationIndex::Surface => cx.theme().colors().surface_background,
+            ElevationIndex::EditorSurface => cx.theme().colors().editor_background,
+            ElevationIndex::ElevatedSurface => cx.theme().colors().elevated_surface_background,
+            ElevationIndex::Wash => gpui::transparent_black(),
+            ElevationIndex::ModalSurface => cx.theme().colors().elevated_surface_background,
+            ElevationIndex::DraggedElement => gpui::transparent_black(),
+        }
+    }
 }