Group diagnostics by primary

Render primary message above the excerpt and supporting messages as block decorations with a `Below` disposition. This is still super rough.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/editor/src/multi_buffer.rs

@@ -559,6 +559,8 @@ impl MultiBuffer {
 
         self.subscriptions.publish_mut([edit]);
 
+        cx.notify();
+
         id
     }