idle listeners on CountingGovernors

[michaelficarra]

CountingGovernors should have idle listeners. addIdleListener registers a callback and removeIdleListener unregisters it.

  addIdleListener(cb: () => void) {
    this.#idleListeners.push(cb);
  }

  removeIdleListener(cb: () => void) {
    const idx = this.#idleListeners.indexOf(cb);
    if (idx >= 0) {
      this.#idleListeners.splice(idx, 1);
    }
  }

  #notifyIdleListeners() {
    for (const cb of this.#idleListeners) {
      try {
        cb();
      } catch {}
    }
  }

Whenever a token is disposed and there are no more active tokens, all of the currently registered callbacks should be called.