LaTeX reader/writer: change handling of math environments.

Certain environments in LaTeX will trigger math mode and can't
occur within math mode: e.g., `align` or `equation`.  Previously
we "downshifted" these, parsing an `align` environment as a
Math element with `aligned`, and an `equation` environment as a
regular display math element.  With this shift, we put these in
Math inlines but retain the original environments.

This works because:

1) texmath handles these environments just fine,
2) and so does MathJax;
3) when writing LaTeX we detect these environments in Math elements
   and emit them verbatim instead of putting them in `$..$` or
   `$$..$$`.

Closes #9711. Closes #9296.

test/command/macros.md

@@ -61,9 +61,9 @@ x &= y\\
 ^D
 \(5-67\)
 
-\[\begin{aligned}
+\begin{align}
 x &= y\\
-\end{aligned}\]
+\end{align}
 
 \emph{hi}