state: throttle batched event processing when input is available

before, calling vim.schedule() from inside an event would execute
the scheduled callback immediately after this event without
checking for user input in between. Break event processing
whenever user input or an interrupt is available.

src/nvim/edit.c

@@ -1024,7 +1024,7 @@ static int insert_handle_key(InsertState *s)
     break;
 
   case K_EVENT:       // some event
-    multiqueue_process_events(main_loop.events);
+    state_handle_k_event();
     goto check_pum;
 
   case K_COMMAND:       // some command