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/gpui/src/gpui.rs

@@ -258,14 +258,14 @@ pub trait EventEmitter<E: Any>: 'static {}
 /// can be used interchangeably.
 pub trait BorrowAppContext {
     /// Set a global value on the context.
-    fn set_global<T: 'static>(&mut self, global: T);
+    fn set_global<T: Global>(&mut self, global: T);
 }
 
 impl<C> BorrowAppContext for C
 where
     C: BorrowMut<AppContext>,
 {
-    fn set_global<G: 'static>(&mut self, global: G) {
+    fn set_global<G: Global>(&mut self, global: G) {
         self.borrow_mut().set_global(global)
     }
 }
@@ -287,3 +287,8 @@ impl<T> Flatten<T> for Result<T> {
         self
     }
 }
+
+/// A marker trait for types that can be stored in GPUI's global state.
+///
+/// Implement this on types you want to store in the context as a global.
+pub trait Global: 'static {}