add \notni

[mennucc]

Description

for set membership the Wikipedia page lists four LaTeX commands: \in \ni \notin \notni ; unfortunately unicode-math does not define the latter.

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{STIX Two Math}
\begin{document}
\[
  a \notni b
\]
\end{document}

Further details

possible patch

0001-add-notni.patch.txt