Normalize floatscript math to text

[dginev]

This PR is an extension to the one originally developed in ar5iv-bindings#2.

It adds a single rewrite rule which detects cases of Math elements located anywhere in the final XML and containing:

• only a single top-level floating superscript or subscript
• with shallow/simple content, namely the script contains only XMTok elements

The DefRewrite then proceeds to replace that floating script with an instance of ltx:sup or ltx:sub, also using the font information implied by the script's Box.

As it turns out, a lot of our tests used this technique, and required a change. I also tested on a new TeX file with relevant examples - I haven't checked that in as a test, but I can do so if requested. I will attach the contents here:

Example TeX test

\documentclass{article}
\title{Test}
\author{Ariel Brown${}^{1}$ Chester Dolby${}^{1,2}$\\
${}^{1}$My Org, ${}^{2}$Their Org}
\begin{document}
\maketitle

\section{Normalize to text}
token-only complex float: ${}^{a \times b}$

chemistry floatscripts: ${}^{76}$ Ge

mixed mode to avoid mathrm: 10${}^{11}$M${}_{\odot}$

mixed mode to avoid mathrm (2): W${}^{1,p}$-estimate

mixed mode compound unit: g/cm${}^{3}$

custom notemark: some text${}^{3}$.

careful with the font: n$^{\mathit{th}}$

\section{Keep as-is}
Complex float: ${}^{x^2}$ or ${}^{\sqrt{x}}$

chemistry fully math mode: ${}^{76}\mathrm{Ge}$

\end{document}

Feedback is most welcome.