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/terminal.c

@@ -457,7 +457,7 @@ static int terminal_execute(VimState *state, int key)
     case K_EVENT:
       // We cannot let an event free the terminal yet. It is still needed.
       s->term->refcount++;
-      multiqueue_process_events(main_loop.events);
+      state_handle_k_event();
       s->term->refcount--;
       if (s->term->buf_handle == 0) {
         s->close = true;