Introduce following for assistant panel (#14479)

Release Notes:

- Added support for following into the assistant panel.

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Nathan <nathan@zed.dev>

crates/gpui/src/gpui.rs

@@ -291,6 +291,10 @@ pub trait BorrowAppContext {
     fn update_global<G, R>(&mut self, f: impl FnOnce(&mut G, &mut Self) -> R) -> R
     where
         G: Global;
+    /// Updates the global state of the given type, creating a default if it didn't exist before.
+    fn update_default_global<G, R>(&mut self, f: impl FnOnce(&mut G, &mut Self) -> R) -> R
+    where
+        G: Global + Default;
 }
 
 impl<C> BorrowAppContext for C
@@ -310,6 +314,14 @@ where
         self.borrow_mut().end_global_lease(global);
         result
     }
+
+    fn update_default_global<G, R>(&mut self, f: impl FnOnce(&mut G, &mut Self) -> R) -> R
+    where
+        G: Global + Default,
+    {
+        self.borrow_mut().default_global::<G>();
+        self.update_global(f)
+    }
 }
 
 /// A flatten equivalent for anyhow `Result`s.