Include external custom math commands from YAML header

I have a bunch of custom math commands that I'd like to ba able to use in all my quarto documents.
Is there a way to include custom math commands in the quarto YAML header (for example, by pointing to a .sty file)?

I think we actually need two external .sty files for this.

• One for LaTeX output, with bare math commands:

\newcommand{\e}{\varepsilon}

• And one for mathjax output, surrounded by \( and \):

\(
\newcommand{\e}{\varepsilon}
\)

Including those in the md or qmd file:

---
format:
  pdf:
    include-in-header: path/to/math_commands.sty
  html:
    include-before-body: path/to/mathjax_commands.sty 
---

$$
\e
$$

HTML output:

PDF output: