Replace Unicode / LaTeX conversion table with R script

[nanxstats]

Suggestion from @yihui: https://github.com/Merck/r2rtf/pull/217/files?diff=unified&w=0#r1564779269

The goal is to put the unicode_latex variable in an R script, to replace the current binary form stored in R/sysdata.rda.

This would also avoid the need of the globalVariables() call for unicode_latex as a hack to bypass R CMD check notes.