gpui: Add Global marker trait (#7095)

This should prevent a class of bugs where one queries the wrong type of
global, which results in oddities at runtime.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/theme/src/theme.rs

@@ -60,7 +60,7 @@ pub fn init(themes_to_load: LoadThemes, cx: &mut AppContext) {
         LoadThemes::JustBase => (Box::new(()) as Box<dyn AssetSource>, false),
         LoadThemes::All(assets) => (assets, true),
     };
-    registry::init(assets, cx);
+    ThemeRegistry::set_global(assets, cx);
 
     if load_user_themes {
         ThemeRegistry::global(cx).load_bundled_themes();