Update extension.ts

[yu45020]

update math reg expression to avoid unexpected math environment

For example

\begin{equation}
a = \text{daf $dsa$}
\end{equation}

The math environment is activated after the equation. This is caused by the regular expression that only removes $dsa$. I change the reg expression so that strings inside $ $ are removed, and then strings in between \begin{*} \end{*} are removed.