gpui: Add support for window transparency & blur on macOS (#9610)

This PR adds support for transparent and blurred window backgrounds on
macOS.

Release Notes:

- Added support for transparent and blurred window backgrounds on macOS
([#5040](https://github.com/zed-industries/zed/issues/5040)).
- This requires themes to specify a new `background.appearance` key
("opaque", "transparent" or "blurred") and to include an alpha value in
colors that should be transparent.

<img width="913" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/7547ee2a-e376-4d55-9114-e6fc2f5110bc">
<img width="994" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/b36fbc14-6e4d-4140-9448-69cad803c45a">
<img width="1020" alt="image"
src="https://github.com/zed-industries/zed/assets/2588851/d70e2005-54fd-4991-a211-ed484ccf26ef">

---------

Co-authored-by: Luiz Marcondes <luizgustavodevergennes@gmail.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/theme/src/theme.rs

@@ -27,7 +27,9 @@ pub use schema::*;
 pub use settings::*;
 pub use styles::*;
 
-use gpui::{AppContext, AssetSource, Hsla, SharedString, WindowAppearance};
+use gpui::{
+    AppContext, AssetSource, Hsla, SharedString, WindowAppearance, WindowBackgroundAppearance,
+};
 use serde::Deserialize;
 
 #[derive(Debug, PartialEq, Clone, Copy, Deserialize)]
@@ -158,6 +160,12 @@ impl Theme {
     pub fn appearance(&self) -> Appearance {
         self.appearance
     }
+
+    /// Returns the [`WindowBackgroundAppearance`] for the theme.
+    #[inline(always)]
+    pub fn window_background_appearance(&self) -> WindowBackgroundAppearance {
+        self.styles.window_background_appearance
+    }
 }
 
 pub fn color_alpha(color: Hsla, alpha: f32) -> Hsla {