Please add support for mhchem KaTeX extension

[saf-dmitry]

Is your feature request related to a problem? Please describe.
I very often need to add chemical reactions, like $\ce{C + O2 -> CO2}$, and quantities with units, like $\pu{1.23 kg.m^{-3}}$, to my notebooks.

Describe the solution you'd like
Please load the mhchem KaTeX extension, so it can be used in notebooks.

Describe alternatives you've considered
Since the mhchem extension for KaTeX exists and works just fine (in other environments, which autoload the mhchem extension), no other alternatives considered.

Additional context
The examples above are rendered by GitHub using the mhchem extension for MathJax.

[mbostock]

For what it’s worth, mhchem already works in Observable Framework. You can load it like so:

```js
import tex from "npm:@observablehq/tex";
import "npm:katex/contrib/mhchem/mhchem.js/+esm";
```