Track dirtyness per item (#26237)

This reduces the number of multibuffer syncs when starting the editor
with 80
files open in the Zed repo from 10,000,000 to 100,000 by avoiding
O(n**2)
dirtyness checks.

Release Notes:

- Fixed a beachball when restarting in a large repo with a large number
open files

crates/zed/src/zed.rs

@@ -2112,6 +2112,7 @@ mod tests {
             })
             .unwrap();
         assert!(window_is_edited(window, cx));
+        let weak = editor.downgrade();
 
         // Closing the item restores the window's edited state.
         let close = window
@@ -2127,11 +2128,12 @@ mod tests {
 
         cx.simulate_prompt_answer("Don't Save");
         close.await.unwrap();
-        assert!(!window_is_edited(window, cx));
 
         // Advance the clock to ensure that the item has been serialized and dropped from the queue
         cx.executor().advance_clock(Duration::from_secs(1));
 
+        weak.assert_released();
+        assert!(!window_is_edited(window, cx));
         // Opening the buffer again doesn't impact the window's edited state.
         cx.update(|cx| {
             open_paths(
@@ -2266,6 +2268,8 @@ mod tests {
         assert_eq!(cx.update(|cx| cx.windows().len()), 1);
         assert!(cx.update(|cx| cx.active_window().is_some()));
 
+        cx.run_until_parked();
+
         // When opening the workspace, the window is not in a edited state.
         let window = cx.update(|cx| cx.active_window().unwrap().downcast::<Workspace>().unwrap());
         assert!(window_is_edited(window, cx));