Use \def instead of \newcommand to define custom LaTeX commands

\newcommand doesn't allow overwriting an existing definition. It worked
fine in Jupyter Notebook, which uses MathJax, which apparently doesn't
actually check this. However, it causes errors in VS Code, which uses
KaTeX. It probably also caused errors when converting a notebook to
LaTeX and typesetting it with a proper LaTeX implementation.

\def overwrites any existing definition and so should work in all cases.