RFC: tab-substitution of LaTeX symbols in Emacs julia-mode

Similar to the new REPL and IJulia behavior implemented in #6911, this allows you to type e.g. `\alpha<TAB>` in Emacs and get `α`.   The list of substitutions is the same.   When a Unicode substitution is not performed, the tab character indents as usual, so the change should be fairly unobtrusive.