Allow disk-based diagnostic progress begin/end events to interleave

When multiple saves occur, we can have multiple start events followed by multiple end events. We don't want to update our project diagnostics view until all pending progress is finished.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/gpui/src/app.rs

@@ -992,7 +992,7 @@ impl MutableAppContext {
         })
     }
 
-    fn observe<E, H, F>(&mut self, handle: &H, mut callback: F) -> Subscription
+    pub fn observe<E, H, F>(&mut self, handle: &H, mut callback: F) -> Subscription
     where
         E: Entity,
         E::Event: 'static,