feat: support {equation}, {equation*}, and {split} (#2369)

* Support {equation}, {equation*}, and {split}

* Update screenshots

* Allow {split} at top level

* Move equation column number check to to ParseArray

* Add token to ParseError

* Sharpen parameters passed to parseArray

* Add token information

* Update an {array} spec in screenshotter

* Adjust {array} screenshotter spec

* Make a non-strict error call when {array} argument specifies too few columns.

* Move context checks to a helper function.

test/screenshotter/ss_data.yaml

@@ -15,7 +15,7 @@
 
 Accents: \vec{A}\vec{x}\vec x^2\vec{x}_2^2\vec{A}^2\vec{xA}^2
 AccentsText: |
-    \begin{array}{l}
+    \begin{array}{lccccc}
     \text{\'\i} & \text{\.\i} & \text{\`\i} & \text{\"\i} & \text{\H\i} & \text{\r\i} \\
     \text{\'\j} & \text{\.\j} & \text{\`\j} & \text{\"\j} & \text{\H\j} & \text{\r\j} \\
     \text{\'a} & \text{\.a} & \text{\`a} & \text{\"a} & \text{\H{a}} & \text{\r{a}} \\
@@ -129,6 +129,9 @@ Dots: |
       \cdots;\dots+\dots\int\dots,\dots \\
       \cdots{};\ldots+\ldots\int\ldots,\ldots
     \end{array}
+Equation:
+    tex: \begin{equation}\begin{split}a& =b+c-d \\ & \quad +e-f \\ & =g+h \\ & =i \end{split}\end{equation}
+    display: 1
 Exponents: a^{a^a_a}_{a^a_a}
 ExtensibleArrows: |
     \begin{array}{l}