add custom notification handling

Feature Request

In order for Plotly to better integrate with our design system, we would like to pass in a custom notifier callback.

Relevant code: notifier.js.

We cannot resolve this with CSS alone as there are interactions between notifications. For example, if we fired three consecutive notifications, they would stack. Using CSS alone, integrating with our existing notification framework is not feasible.

Thanks!

从 src/lib/notifier.js 开始，这是 issue 中提到的唯一相关入口点，并跟踪通知是如何创建和堆叠的。确定外部通知框架所需的 callback 接口；当调用方可以提供自定义通知处理，同时连续通知仍能协同工作时，即表示完成。