[FEATURE] Custom latex commands

[Batres3]

Description of the Problem

It is generally very useful in latex to be able to define custom commands (like \Li{}{}) for the polylogarithm functions) and it is a feature I use widely in regular latex.

Description of the Solution

Add a section to the settings (like the one currently available for snippets) where one can simply add calls to \newcommand and then have those used globally in obsidian