Fix server shut down on exit.

If there's a websocket being waited on the shut down won't actually
happen. Add a new event for shut down and send it to break the websocket
out of its loop.

game/sim/gameupdateevents.py

@@ -25,6 +25,7 @@ class GameUpdateEvents:
     deleted_flights: set[UUID] = field(default_factory=set)
     selected_flight: UUID | None = None
     deselected_flight: bool = False
+    shutting_down: bool = False
 
     @property
     def empty(self) -> bool:
@@ -89,3 +90,7 @@ class GameUpdateEvents:
         self.deselected_flight = True
         self.selected_flight = None
         return self
+
+    def shut_down(self) -> GameUpdateEvents:
+        self.shutting_down = True
+        return self