pgfplots legend regression

[dginev]

This was a well-hidden regression as we don't have pgfplots tests in place.

Minimally, it is reproduced by the error from converting the snippet:

\documentclass{article}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}

\begin{document}
\begin{tikzpicture}
\begin{groupplot}
  \nextgroupplot[legend to name = {complegend}]
\end{groupplot}
\end{tikzpicture}%
\end{document}

Which has regressed to emit an error:

Error:undefined:\pgfplots@legend@to@name The token T_CS[\pgfplots@legend@to@name] is not defined. at min.tex; line 17 col 0

which can then lead to a Fatal-grade error cascade, as with converting arXiv:1709.07394.

I had a careful bisecting session and traced back the original cause to the Scan Toks feature commit https://github.com/brucemiller/LaTeXML/commit/4757b39ddd0977099ef7c640ce546206adf735d6 . In my testing this variation has been broken in all commits since July 2023, so it isn't related to any recent PR work.

Tested with texlive 2022, but I suspect this is generic enough to be reproducible on other texlives.