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/src/extension_events.rs

@@ -0,0 +1,35 @@
+use gpui::{App, AppContext as _, Context, Entity, EventEmitter, Global, ReadGlobal as _};
+
+pub fn init(cx: &mut App) {
+    let extension_events = cx.new(ExtensionEvents::new);
+    cx.set_global(GlobalExtensionEvents(extension_events));
+}
+
+struct GlobalExtensionEvents(Entity<ExtensionEvents>);
+
+impl Global for GlobalExtensionEvents {}
+
+/// An event bus for broadcasting extension-related events throughout the app.
+pub struct ExtensionEvents;
+
+impl ExtensionEvents {
+    /// Returns the global [`ExtensionEvents`].
+    pub fn global(cx: &App) -> Entity<Self> {
+        GlobalExtensionEvents::global(cx).0.clone()
+    }
+
+    fn new(_cx: &mut Context<Self>) -> Self {
+        Self
+    }
+
+    pub fn emit(&mut self, event: Event, cx: &mut Context<Self>) {
+        cx.emit(event)
+    }
+}
+
+#[derive(Clone)]
+pub enum Event {
+    ExtensionsUpdated,
+}
+
+impl EventEmitter<Event> for ExtensionEvents {}