Custom Macros for Writing Markdown in VS Code

TL;DR: I want to write (basic) custom macros that VS Code can render in markdown files

I use VS Code with the Markdown Preview Enchanced extension to write mathematical documents in Markdown. I often include basic (La)TeX commands in them, eg $\bar x = \tfrac1n \sum_i x_i$. In the past, I was able to include custom macros, such as \eps in place of \varepsilon. VS Code's preview then renders these correctly. I can't for the life of me remember how I did this, though, and I've moved to a new computer.

I still have a file on my old computer called config.js, which consists of a whole list of these. But, it's just in a documents folder. I presume I made a copy of it, to remember what macros I'd made, in case some update deleted it. I just can't see where I should put that file on my new computer. Here's an abbreviated copy of the file.

({
  katexConfig: {
  "macros": {}
},
  mathjaxConfig: {
  "tex": { macros: {
    ...
    eps: "\\varepsilon",
    ...
  }},
  "options": {},
  "loader": {}
},
})

Also, I'd slightly prefer to use KaTeX to MathJax, but that's far from a deal-breaker.

Incidentally, I know you can do things like add $\newcommand{\eps}{\varepsilon}$, but that isn't what I'm looking for.