Lazy load the js

还没有人认领这个 Issue。

## Describe the issue

I don't think that the js is loaded lazily which increases the time until the jlab splash screen shows up.

## Proposed Solution

Lazy load the widgetExports similar to https://github.com/QuantStack/ipycytoscape/pull/222

Though this may be complicated by the reading of package.json for the version? https://github.com/matplotlib/ipympl/blob/3a75b2bbab58bf5748c87c96c05e79e6545f64e0/js/src/index.js#L4

比较 ipycytoscape 的 pull request 222 中的懒加载方式，然后检查 ipympl 的 js/src/index.js 以及 package.json 中的版本处理。确定如何在不破坏版本访问的情况下懒加载 widgetExports；完成的标准是 JupyterLab 启动画面可以更早出现，同时 widget 仍能正确初始化。

由索引模型根据 Issue 内容生成。