Load JSON themes (#6893)

This PR changes the theme loading to use the JSON themes bundled with
the binary rather then the Rust theme definitions.

### Performance

I profiled this using `cargo run --release` to see what the speed
differences would be now that we're deserializing JSON:

**Before:** `ThemeRegistry::load_user_themes` took 16.656666ms
**After:** `ThemeRegistry::load_user_themes` took 18.784875ms

It's slightly slower, but not by much. There is probably some work we
could do here to bring down the theme loading time in general.

Release Notes:

- N/A

crates/theme/src/theme.rs

@@ -33,7 +33,7 @@ pub use styles::*;
 pub use themes::*;
 pub use user_theme::*;
 
-use gpui::{AppContext, Hsla, SharedString};
+use gpui::{AppContext, AssetSource, Hsla, SharedString};
 use serde::Deserialize;
 
 #[derive(Debug, PartialEq, Clone, Copy, Deserialize)]
@@ -51,7 +51,6 @@ impl Appearance {
     }
 }
 
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub enum LoadThemes {
     /// Only load the base theme.
     ///
@@ -59,15 +58,20 @@ pub enum LoadThemes {
     JustBase,
 
     /// Load all of the built-in themes.
-    All,
+    All(Box<dyn AssetSource>),
 }
 
 pub fn init(themes_to_load: LoadThemes, cx: &mut AppContext) {
-    cx.set_global(ThemeRegistry::default());
-    match themes_to_load {
-        LoadThemes::JustBase => (),
-        LoadThemes::All => cx.global_mut::<ThemeRegistry>().load_user_themes(),
+    let (assets, load_user_themes) = match themes_to_load {
+        LoadThemes::JustBase => (Box::new(()) as Box<dyn AssetSource>, false),
+        LoadThemes::All(assets) => (assets, true),
+    };
+    cx.set_global(ThemeRegistry::new(assets));
+
+    if load_user_themes {
+        cx.global_mut::<ThemeRegistry>().load_user_themes();
     }
+
     ThemeSettings::register(cx);
 
     let mut prev_buffer_font_size = ThemeSettings::get_global(cx).buffer_font_size;