add \overunderset to amsmath.sty

[dginev]

Tested with arXiv:2511.05230v1.

This was reported at arXiv/html_feedback#5230

The PR simply combines the existing definitions of \overset and \underset. The resulting presentation MathML is a little suspicious however. For the minimal example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ \overunderset{\text{\rm a.s.}}{\varepsilon\to0}{\longrightarrow} \]
\end{document}

The expected behavior is the long right arrow to span both over- and under-accents. In today's Chrome that isn't the case, and the MathML sizes are seemingly rather off. LaTeXML (with this PR) produces:

  <munderover accent="true" accentunder="true">
    <mo stretchy="false">⟶</mo>
    <mrow>
      <mi mathsize="1.420em">ε</mi>
      <mo mathsize="1.420em" stretchy="false">→</mo>
      <mn mathsize="1.420em">0</mn>
    </mrow>
    <mtext>a.s.</mtext>
  </munderover>

So I welcome thoughts on where may be the best place to understand/adjust the sizing details - and stretchy details. Encouragement to do a deeper dive is welcome too.