Support {gather} and {gather*} (#2183)

* Support `gather` and `gather*`

* Add bogus screenshots

* Add correct screenshots

* Fix typo

* Update screenshots

* Improve error handling

* Scope styles inside .katex{}

* Revert console.log to console.error

test/katex-spec.js

@@ -2726,6 +2726,27 @@ describe("An aligned environment", function() {
     });
 });
 
+describe("A gather environment", function() {
+    const nonStrictDisplay = new Settings({displayMode: true, strict: false});
+
+    it("should fail outside display mode", () => {
+        expect`\begin{gather}a+b\\c+d\end{gather}`.not.toParse(nonstrictSettings);
+    });
+
+    it("should fail outside display mode", () => {
+        expect`\begin{gather*}a+b\\c+d\end{gather*}`.not.toParse(nonstrictSettings);
+    });
+
+    it("should build if in display mode", () => {
+        expect`\begin{gather}a+b\\c+d\end{gather}`.toBuild(nonStrictDisplay);
+    });
+
+    it("should build if in display mode", () => {
+        expect`\begin{gather*}a+b\\c+d\end{gather*}`.toBuild(nonStrictDisplay);
+    });
+
+});
+
 describe("operatorname support", function() {
     it("should not fail", function() {
         expect("\\operatorname{x*Π∑\\Pi\\sum\\frac a b}").toBuild();

test/screenshotter/ss_data.yaml

@@ -141,6 +141,9 @@ FractionTest: |
          \xrightarrow[\dfrac g h]{\displaystyle\frac g h} \; \xrightarrow [2.\, \dfrac c d] {1.\, \displaystyle\frac c d}
     \end{array}
 Functions: \sin\cos\tan\ln\log
+Gather:
+    tex: \begin{gather} a=\frac 1 2 \\ e=b+c \end{gather}
+    display: 1
 Gathered: |
     \begin{gathered}
       x = \frac{1}{2} \\