fix: mock v2 api of MathJax

[maartenbreddels]

This is a workaround or fix for #516

There are a few problems:

• We wait for the widgets to be build before we render mathjax, however, if widgets are inside a VBox, they may be rendered async, so they come after mathjax is done.
• typeset in https://github.com/jupyter-widgets/ipywidgets/blob/b3708e1305248b132999322266aab07c7e9ec3dc/packages/controls/src/utils.ts#L16 (which is called by description widget in https://github.com/jupyter-widgets/ipywidgets/blob/b3708e1305248b132999322266aab07c7e9ec3dc/packages/controls/src/widget_description.ts#L62) uses the v2 API of MathJax, while we use v3 since Bundle mathjax with Voilà #470 (since voila 0.1.18)

In this PR I mock the v2 API a bit, and rerender all math. I am not sure we should do this.

cc @jasongrout
We could go mathjax v3 on all platforms, notebook, lab, widgets, or revert to v2 mathjax in voila.

[jasongrout]

The JLab issue for upgrading to MathJax 3 is jupyterlab/jupyterlab#7218

The non-core extension to upgrade to MathJax 3 is https://github.com/jupyterlab/jupyter-renderers/tree/master/packages/mathjax3-extension

[maartenbreddels]

Great, that example is useful, it wasn't easy figuring out the API.

[maartenbreddels]

Might be good to take a look at some of the commits in widgetti/solara#155 where I continued this work.