Report unsupported macros by katex and expand user-define commands

Break up a latex source by divisions (`\section` , `\subsection`, etc.) and turn those divisions into environments. E.g., `\section{foo}XXX\section{bar}YYY` becomes 

```
\begin{_section}[foo]XXX\end{_section}
\begin{_section}[bar]YYY\end{_section}
```

This is an intermediate step in the PreTeXt conversion.

Merge main

Rebase

Add function to extract information from the `\author{...}` tag that appears before `\begin{document}...`

Necessary macro subs and environment subs to create valid PreTeXt from LaTeX.

* started adding pretext package to cli