Make \overset and friends mark mo as non-accents (#2800)

Davide P. Cervone
2023-03-19T19:07:48Z

If the top or bottom of \overset, \underset or \overunderset is a single mo element whose operator-dictionary entry indicates it is an accent by default, then the output is incorrect (the display uses accent size and spacing, and accent substitution can be performed). This PR resolves that buy explicitly marking mo elements with accent="false". This will be filtered out if that is the operator dictionary default, but will be retained otherwise.

Resolves issue #2800.

Companion to mathjax/MathJax-src#763.