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/default_theme.rs

@@ -1,5 +1,7 @@
 use std::sync::Arc;
 
+use gpui::WindowBackgroundAppearance;
+
 use crate::prelude::*;
 
 use crate::{
@@ -15,6 +17,7 @@ fn zed_pro_daylight() -> Theme {
         name: "Zed Pro Daylight".into(),
         appearance: Appearance::Light,
         styles: ThemeStyles {
+            window_background_appearance: WindowBackgroundAppearance::Opaque,
             system: SystemColors::default(),
             colors: ThemeColors::light(),
             status: StatusColors::light(),
@@ -45,6 +48,7 @@ pub(crate) fn zed_pro_moonlight() -> Theme {
         name: "Zed Pro Moonlight".into(),
         appearance: Appearance::Dark,
         styles: ThemeStyles {
+            window_background_appearance: WindowBackgroundAppearance::Opaque,
             system: SystemColors::default(),
             colors: ThemeColors::dark(),
             status: StatusColors::dark(),