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/982.md

@@ -7,5 +7,9 @@
 y=x^2
 \EEQ
 ^D
-[ Para [ Math DisplayMath "y=x^2" ] ]
+[ Para
+    [ Math
+        DisplayMath "\\begin{equation}\ny=x^2\n\\end{equation}"
+    ]
+]
 ```