internal/warpc: Enable KaTeX mhchem extension

[jmooring]

I ran into this limitation on the docs site while implementing math rendering with the transform.ToMath function instead of client-side rendering as we did previously. With client-side MathJax, chemistry support was included which is why we have this example:

https://gohugo.io/content-management/mathematics/#chemistry

This also bumps KaTeX to the latest version.

[bep]

I will merge this, but my near future goal is to get some kind of hugo pack command that would allow us/others to build Wasm files for Hugo that they could import via Hugo modules (for math, but I guess other stuff down the line).