add onlyPreamble checks for \documentclass and \documentstyle

[dginev]

Add onlyPreamble guards for class-selection LaTeX macros. The absence of these errors became apparent during an Authorea debugging session, so I'm contributing them back upstream.

The goal is that given input such as:

\documentclass[11pt,letter]{article}


\begin{document}

\documentclass[letterpaper,10pt]{article}
...
\end{document}

We would like LaTeXML to report a preamble violation error as in:

Error:unexpected:\documentclass The current command '\documentclass' can only appear in the preamble

This is all setup already for usepackage, so just leveraging that infrastructure in this PR.

Bonus: My sublime editing environment removes trailing whitespace (that is actually a tiny speedup for RawTeX I believe)

[brucemiller]

Thanks!