Markup for M2 output in Emacs

[mahrud]

Quoting from @d-torrance in Macaulay2/M2#3610 (comment):

Tangentially, it would be great if M2-mode could replace strings wrapped in $ .. $ with rudimentary tex, but this may be nontrivial.

It can! See https://github.com/Macaulay2/M2/wiki/Previewing-TeX-in-Macaulay2-Emacs-buffers

This is useful, but I think has a few drawbacks:

1. we don't want typing `print "$ blah $" in the input line to produce LaTeX
2. we don't want an output like use the synonym Cremona$point to produce LaTeX

And a big missing features:
3. maybe we need a new topLevelMode similar to WebApp that produces LaTeX instead of html.