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/storybook/src/storybook.rs

@@ -69,7 +69,7 @@ fn main() {
             .unwrap();
         cx.set_global(store);
 
-        theme::init(theme::LoadThemes::All, cx);
+        theme::init(theme::LoadThemes::All(Box::new(Assets)), cx);
 
         let selector = story_selector;