fix(napi): don't hold on to borrow during iteration (denoland#17461)

I mistakenly held on to a RefCell's borrow for the whole time of
iteration, but since these counters can be refed/unrefed from any 
thread that is a mistake.

ext/napi/lib.rs

@@ -533,7 +533,8 @@ pub fn init<P: NapiPermissions + 'static>(unstable: bool) -> Extension {
  maybe_scheduling = true;
  }
 
- for (_id, counter) in napi_state.tsfn_ref_counters.borrow().iter() {
+ let tsfn_ref_counters = napi_state.tsfn_ref_counters.borrow().clone();
+ for (_id, counter) in tsfn_ref_counters.iter() {
  if counter.load(std::sync::atomic::Ordering::SeqCst) > 0 {
  maybe_scheduling = true;
  break;