gpui: Add Global marker trait (#7095)

This should prevent a class of bugs where one queries the wrong type of
global, which results in oddities at runtime.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/workspace/src/notifications.rs

@@ -1,7 +1,7 @@
 use crate::{Toast, Workspace};
 use collections::HashMap;
 use gpui::{
-    AnyView, AppContext, AsyncWindowContext, DismissEvent, Entity, EntityId, EventEmitter,
+    AnyView, AppContext, AsyncWindowContext, DismissEvent, Entity, EntityId, EventEmitter, Global,
     PromptLevel, Render, Task, View, ViewContext, VisualContext, WindowContext,
 };
 use std::{any::TypeId, ops::DerefMut};
@@ -39,6 +39,8 @@ pub(crate) struct NotificationTracker {
     notifications_sent: HashMap<TypeId, Vec<usize>>,
 }
 
+impl Global for NotificationTracker {}
+
 impl std::ops::Deref for NotificationTracker {
     type Target = HashMap<TypeId, Vec<usize>>;