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/storybook/src/storybook.rs

@@ -76,7 +76,7 @@ fn main() {
 
         let selector = story_selector;
 
-        let theme_registry = ThemeRegistry::global(cx);
+        let theme_registry = <dyn ThemeRegistry>::global(cx);
         let mut theme_settings = ThemeSettings::get_global(cx).clone();
         theme_settings.active_theme = theme_registry.get(&theme_name).unwrap();
         ThemeSettings::override_global(theme_settings, cx);