Investigate `pExpr` in the Markdown printer

[BilalM04]

Current state of pExpr is detailed in https://github.com/JacquesCarette/Drasil/pull/3823#discussion_r1658784453.

pExpr re-defines all the patterns that are different, then delegates to the TeX function in a catch all statement. But since the pExpr function involves recursive calls, the patterns which make a recursive call are redefined as well, leading to some duplication. This is because the current TeX.pExpr does not account for variabilities in the LaTeX commands.

The pExpr function in the Markdown printer needs to be revisited to avoid duplication. Additionally, the TeX.pExpr function needs to be revisited to account for variabilities in syntax.