Support {smallmatrix}, {subarray}, and \substack (#1969)

* Support {smallmatrix}, {subarray}, and \substack

* Fix lint errors

* Update docs. Screenshots step 1.

* Screenshots step 2

* Screenshots step 3

* Pick up review comments

* Fixed arraycolsep for {smallmatrix}

* Fixed lint error

* Updated screenshots

test/katex-spec.js

@@ -2582,6 +2582,43 @@ describe("An array environment", function() {
 
 });
 
+describe("A subarray environment", function() {
+
+    it("should accept only a single alignment character", function() {
+        const parse = getParsed`\begin{subarray}{c}a \\ b\end{subarray}`;
+        expect(parse[0].type).toBe("array");
+        expect(parse[0].cols).toEqual([
+            {type: "align", align: "c"},
+        ]);
+        expect`\begin{subarray}{cc}a \\ b\end{subarray}`.not.toParse();
+        expect`\begin{subarray}{c}a & b \\ c & d\end{subarray}`.not.toParse();
+        expect`\begin{subarray}{c}a \\ b\end{subarray}`.toBuild();
+    });
+
+});
+
+describe("A substack function", function() {
+
+    it("should build", function() {
+        expect`\sum_{\substack{ 0<i<m \\ 0<j<n }}  P(i,j)`.toBuild();
+    });
+    it("should accommodate spaces in the argument", function() {
+        expect`\sum_{\substack{ 0<i<m \\ 0<j<n }}  P(i,j)`.toBuild();
+    });
+    it("should accommodate macros in the argument", function() {
+        expect`\sum_{\substack{ 0<i<\varPi \\ 0<j<\pi }}  P(i,j)`.toBuild();
+    });
+
+});
+
+describe("A smallmatrix environment", function() {
+
+    it("should build", function() {
+        expect`\begin{smallmatrix} a & b \\ c & d \end{smallmatrix}`.toBuild();
+    });
+
+});
+
 describe("A cases environment", function() {
 
     it("should parse its input", function() {

test/screenshotter/ss_data.yaml

@@ -37,7 +37,9 @@ Arrays: |
         1&2&3\\ \hline
         1+1&2+1&3+1\cr1\over2&\scriptstyle 1/2&\frac12\\[1ex] \hline \hdashline
         \begin{pmatrix}x\\y\end{pmatrix}&0&\begin{vmatrix}a&b\\c&d\end{vmatrix}
-    \end{array}\right]
+    \end{array}\right] \\
+    \begin{smallmatrix} a & b \\ c & d \end{smallmatrix} \;\;
+    \begin{subarray}{c}a \\ b\end{subarray}
 ArrayMode:
     tex: |
         \begin{matrix}
@@ -270,7 +272,8 @@ OpLimits: |
     \begin{matrix}
         {\sin_2^2 \lim_2^2 \int_2^2 \sum_2^2}
         {\displaystyle \lim_2^2 \int_2^2 \intop_2^2 \sum_2^2} \\
-        \limsup_{x \rightarrow \infty} x \stackrel{?}= \liminf_{x \rightarrow \infty} x
+        \limsup_{x \rightarrow \infty} x \stackrel{?}= \liminf_{x \rightarrow \infty} x \\
+        \displaystyle \sum_{\substack{0<i<m\\0<j<n}}
     \end{matrix}
 OverUnderline: x\underline{x}\underline{\underline{x}}\underline{x_{x_{x_x}}}\underline{x^{x^{x^x}}}\overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{\underline{x}}\underline{\overline{x}}}
 OverUnderset: |