Extract auto_update_ui crate (#21008)

This PR extracts an `auto_update_ui` crate out of the `auto_update`
crate.

This allows `auto_update` to not depend on heavier crates like `editor`,
which in turn allows other downstream crates to start building sooner.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -367,6 +367,7 @@ fn main() {
         AppState::set_global(Arc::downgrade(&app_state), cx);
 
         auto_update::init(client.http_client(), cx);
+        auto_update_ui::init(cx);
         reliability::init(
             client.http_client(),
             system_id.as_ref().map(|id| id.to_string()),

crates/zed/src/zed.rs

@@ -223,7 +223,7 @@ pub fn initialize_workspace(
             status_bar.add_right_item(cursor_position, cx);
         });
 
-        auto_update::notify_of_any_new_update(cx);
+        auto_update_ui::notify_of_any_new_update(cx);
 
         let handle = cx.view().downgrade();
         cx.on_window_should_close(move |cx| {