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/zed/src/zed.rs

@@ -1139,7 +1139,7 @@ mod tests {
         path::{Path, PathBuf},
         time::Duration,
     };
-    use theme::{ThemeRegistry, ThemeSettings};
+    use theme::{RealThemeRegistry, ThemeRegistry, ThemeSettings};
     use workspace::{
         item::{Item, ItemHandle},
         open_new, open_paths, pane, NewFile, OpenVisible, SaveIntent, SplitDirection,
@@ -3419,7 +3419,7 @@ mod tests {
                     .unwrap(),
             ])
             .unwrap();
-        let themes = ThemeRegistry::default();
+        let themes = RealThemeRegistry::default();
         settings::init(cx);
         theme::init(theme::LoadThemes::JustBase, cx);