feat: support custom katex instance

[jackykwandesign]

There is no way to support katex extension, e.g. https://github.com/KaTeX/KaTeX/tree/main/contrib/mhchem
One fork implement mech so we can support iktakahiro@5875bbb

Before:

After:

It will be great if any better approach

[jackykwandesign]

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[mjbvz]

Thanks!