... you were excellent at making the syntax highlighting modules for Notepad++ ...
Uhh, wish I could take the honor, but the highlighting modules are already coded by the N++ developers. No coding is needed for the N++ highlighter, it's more or less just creating the keyword lists, the operators list, the comment and string delimiters and set the desired colors and text styles. All this is nicely collected into an XML file which you provide to N++.
Doing the whole coding in JavaScript is a completly different task, and to be honest I haven't coded in JS since v1.1 of the language, which is how long, 20+ years ago. So all the new stuff of JS incl. DOM (which is certainly required to alterate the appearence/content of a webpage) is far beyond my scope of the language.
But what's about the codebox highlighter of this forum, coudn't it be adapted/used for Discord as well?