Convert query capture indices to style ids

* Introduce a Theme struct as a new part of the app's settings
* Store on each Language a ThemeMap, which converts the capture ids
  from that language's highlight query into StyleIds, which identify
  styles in the current Theme.
* Update `highlighted_chunks` methods to provide StyleIds instead of
  capture ids.

zed/src/main.rs

@@ -18,6 +18,7 @@ fn main() {
 
     let (_, settings) = settings::channel(&app.font_cache()).unwrap();
     let language_registry = Arc::new(language::LanguageRegistry::new());
+    language_registry.set_theme(&settings.borrow().theme);
     let app_state = AppState {
         language_registry,
         settings,