LaTeX reader: fix spurious paragraph breaks in math environments

[emmanuel-ferdman]

PR Summary

When trailing spaces appeared before \end{equation} or \end{align}, pandoc incorrectly inserted blank lines in the output, breaking the math environment. This occurred because mathEnv used stripTrailingNewlines which only removed newlines but left spaces. The fix changes to trimr which removes all trailing whitespace (spaces, tabs, and newlines).

Example input:

\begin{equation}
 a
 \end{equation}

Outpt before (broken):

\begin{equation}
 a

\end{equation}

Spurious blank line creates paragraph break
Output after (fixed):

\begin{equation}
 a
\end{equation}

Note: The added tests may be a bit too specific, but I included them for completeness. Let me know if we should drop them.

Fixes #11257.

[jgm]

Looks good, thanks!