Add end of service notifications (#30982)

Release Notes:

- N/A

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/gpui/src/util.rs

@@ -27,6 +27,19 @@ pub trait FluentBuilder {
         self.map(|this| if condition { then(this) } else { this })
     }
 
+    /// Conditionally modify self with the given closure.
+    fn when_else(
+        self,
+        condition: bool,
+        then: impl FnOnce(Self) -> Self,
+        else_fn: impl FnOnce(Self) -> Self,
+    ) -> Self
+    where
+        Self: Sized,
+    {
+        self.map(|this| if condition { then(this) } else { else_fn(this) })
+    }
+
     /// Conditionally unwrap and modify self with the given closure, if the given option is Some.
     fn when_some<T>(self, option: Option<T>, then: impl FnOnce(Self, T) -> Self) -> Self
     where