Fix equation gathering

[jojo2357]

Fix #3321

Previously, the \item{\[a=b\]} was returning true from isDisplayMath due to it not caring about the depth that the DisplayMath child was. This fix passed all tests locally by requiring the math child to be at a depth of exactly 1.

What should this do?

\begin{enumerate}
    \item{ \[a=b\] \[b=a\] }
\end{enumerate}

right now it does not error and there is no test for it.