extension: Add ExtensionEvents for listening to extension-related events (#26562)

This PR adds a new `ExtensionEvents` event bus that can be used to
listen for extension-related events throughout the app.

Today you need to have a handle to the `ExtensionStore` (which entails
depending on `extension_host`) in order to listen for extension events.

With this change subscribers only need to depend on `extension`, which
has a leaner dependency graph.

Release Notes:

- N/A

crates/extension_host/src/extension_store_test.rs

@@ -780,6 +780,7 @@ fn init_test(cx: &mut TestAppContext) {
         let store = SettingsStore::test(cx);
         cx.set_global(store);
         release_channel::init(SemanticVersion::default(), cx);
+        extension::init(cx);
         theme::init(theme::LoadThemes::JustBase, cx);
         Project::init_settings(cx);
         ExtensionSettings::register(cx);