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/10160.md

@@ -7,5 +7,9 @@
 \begin{table}[h]
 \end{table}
 ^D
-[ Para [ Math DisplayMath "[0,1)" ] ]
+[ Para
+    [ Math
+        DisplayMath "\\begin{equation}\n  [0,1)\n\\end{equation}"
+    ]
+]
 ```

test/command/3113.md

@@ -11,7 +11,7 @@ E&=&F
 [ Para
     [ Math
         DisplayMath
-        "\\begin{aligned}\nA&=&B,\\\\\nC&=&D,\\\\\n%\\end{eqnarray}\n%\\begin{eqnarray}\nE&=&F\n\\end{aligned}"
+        "\\begin{eqnarray}\nA&=&B,\\\\\nC&=&D,\\\\\n%\\end{eqnarray}\n%\\begin{eqnarray}\nE&=&F\n\\end{eqnarray}"
     ]
 ]
 ```

test/command/7512.md

@@ -4,7 +4,12 @@
 [d,\delta]=0.
 \end{equation*}
 ^D
-[ Para [ Math DisplayMath "[d,\\delta]=0." ] ]
+[ Para
+    [ Math
+        DisplayMath
+        "\\begin{equation*}\n[d,\\delta]=0.\n\\end{equation*}"
+    ]
+]
 ```
 
 ```

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}"
+    ]
+]
 ```

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}
 

test/command/refs.md

@@ -118,7 +118,7 @@ Accuracy~\eqref{eq:Accuracy} is the proportion, measuring true results among all
 , Para
     [ Math
         DisplayMath
-        "\\label{eq:Accuracy}\n  Accuracy = \\frac{t_p + t_n}{t_p + f_p + f_n + t_n}"
+        "\\begin{equation}\n  \\label{eq:Accuracy}\n  Accuracy = \\frac{t_p + t_n}{t_p + f_p + f_n + t_n}\n\\end{equation}"
     ]
 ]
 ```