theme: Turn ThemeRegistry into a trait (#20076)

This PR converts the `ThemeRegistry` type into a trait instead of a
concrete implementation.

This allows for the extension store to depend on an abstraction rather
than the concrete theme registry implementation.

We currently have two `ThemeRegistry` implementations:

- `RealThemeRegistry` — this was previously the `ThemeRegistry` and
contains the real implementation of the registry.
- `VoidThemeRegistry` — a null object that doesn't have any behavior.

Release Notes:

- N/A

crates/settings_ui/src/appearance_settings_controls.rs

@@ -83,7 +83,7 @@ impl RenderOnce for ThemeControl {
             "theme",
             value.clone(),
             ContextMenu::build(cx, |mut menu, cx| {
-                let theme_registry = ThemeRegistry::global(cx);
+                let theme_registry = <dyn ThemeRegistry>::global(cx);
 
                 for theme in theme_registry.list_names(false) {
                     menu = menu.custom_entry(