Extend support for unicode chars in LaTeX

[SuzanneSoy]

@${xᵢ} and similar unicode math has high chances of not being supported in LaTeX (and renders differently in MathJax than @${x_i}.

We should pre-process the unicode chars and replace them with the corresponding LaTeX sequence (unless the user specifies not to do so).

You may be aware of this already, but the tables from the unicode-math package and the LaTeX Unicode Character References may be useful, with the unicode-math table being more recently updated.

(I'm actually only here to report a typo.)

[SuzanneSoy]

@whatsthepoint I know about unicode-math, but it only support a few characters, so you need to write your own mappings in a lot of cases, e.g. $xᵢ$ and $x₉$ are most likely not working with unicode-math. I already have a small table of extra mappings, I just need to clean it up and copy it over here.

Thanks for the LaTeX Unicode Character References, I didn't know about it. There seem to be some mapping tables there that could be useful :) .