Make block_with_timeout more robust (#11670)

The previous implementation relied on a background thread to wake up the
main thread,
which was prone to priority inversion under heavy load.

In a synthetic test, where we spawn 200 git processes while doing a 5ms
timeout, the old version blocked for 5-80ms, the new version blocks for
5.1-5.4ms.

Release Notes:

- Improved responsiveness of the main thread under high system load

crates/gpui/src/platform/mac/dispatcher.rs

@@ -87,12 +87,13 @@ impl PlatformDispatcher for MacDispatcher {
         }
     }
 
-    fn tick(&self, _background_only: bool) -> bool {
-        false
-    }
-
-    fn park(&self) {
-        self.parker.lock().park()
+    fn park(&self, timeout: Option<Duration>) -> bool {
+        if let Some(timeout) = timeout {
+            self.parker.lock().park_timeout(timeout)
+        } else {
+            self.parker.lock().park();
+            true
+        }
     }
 
     fn unparker(&self) -> Unparker {