Add a noteenv environment

[kno10]

Because \note{} is a command, certain constructs such as verbatim cannot be used inside.
It would be good to add a noteenv (c.f., onlyenv, visibleenv, etc.) to allow

\begin{noteenv}
\begin{verbatim}
verbatim
\end{verbatim}
\end{noteenv}

See also #496