Refactor workspace notifications to use explicit NotificationId type (#10342)

This PR reworks the way workspace notifications are identified to use a
new `NotificationId` type.

A `NotificationId` is bound to a given type that is used as a unique
identifier. Generally this will be a unit struct that can be used to
uniquely identify this notification.

A `NotificationId` can also accept an optional `ElementId` in order to
distinguish between different notifications of the same type.

This system avoids the issue we had previously of selecting `usize` IDs
somewhat arbitrarily and running the risk of having two independent
notifications collide (and thus interfere with each other).

This also fixes a bug where multiple suggestion notifications for the
same extension could be live at once

Fixes https://github.com/zed-industries/zed/issues/10320.

Release Notes:

- Fixed a bug where multiple extension suggestions for the same
extension could be shown at once
([#10320](https://github.com/zed-industries/zed/issues/10320)).

---------

Co-authored-by: Max <max@zed.dev>

crates/vcs_menu/src/lib.rs

@@ -13,6 +13,7 @@ use ui::{
     LabelSize, ListItem, ListItemSpacing, Selectable,
 };
 use util::ResultExt;
+use workspace::notifications::NotificationId;
 use workspace::{ModalView, Toast, Workspace};
 
 actions!(branches, [OpenRecent]);
@@ -125,9 +126,11 @@ impl BranchListDelegate {
     }
 
     fn display_error_toast(&self, message: String, cx: &mut WindowContext<'_>) {
-        const GIT_CHECKOUT_FAILURE_ID: usize = 2048;
         self.workspace.update(cx, |model, ctx| {
-            model.show_toast(Toast::new(GIT_CHECKOUT_FAILURE_ID, message), ctx)
+            struct GitCheckoutFailure;
+            let id = NotificationId::unique::<GitCheckoutFailure>();
+
+            model.show_toast(Toast::new(id, message), ctx)
         });
     }
 }