Support \widecheck (#1406)

Fixes #1221.

Implemented as an upside-down version of `\widehat`.

test/katex-spec.js

@@ -1865,6 +1865,7 @@ describe("An accent parser", function() {
 
     it("should parse stretchy, shifty accents", function() {
         expect("\\widehat{x}").toParse();
+        expect("\\widecheck{x}").toParse();
     });
 
     it("should parse stretchy, non-shifty accents", function() {
@@ -1892,6 +1893,7 @@ describe("An accent builder", function() {
 describe("A stretchy and shifty accent builder", function() {
     it("should not fail", function() {
         expect("\\widehat{AB}").toBuild();
+        expect("\\widecheck{AB}").toBuild();
         expect("\\widehat{AB}^2").toBuild();
         expect("\\widehat{AB}_2").toBuild();
         expect("\\widehat{AB}_2^2").toBuild();

test/screenshotter/ss_data.yaml

@@ -301,7 +301,7 @@ StretchyAccent: |
     \begin{array}{l}
         \overrightarrow{AB} \quad \overleftarrow{AB} \quad \Overrightarrow{AB} \quad \overleftrightarrow{AB} \quad \overgroup{AB} \\
         \overlinesegment{AB} \quad \overleftharpoon{AB} \quad \overrightharpoon{AB} \quad \color{red}{\overrightarrow{AB}} \quad \widehat{\theta} \widetilde{A} \\
-        \widehat{AB} \quad \widehat{ABC} \quad \widetilde{AB} \quad \widetilde{ABC} \\
+        \widecheck{AB} \quad \widehat{ABC} \quad \widetilde{AB} \quad \widetilde{ABC} \\
         \overrightarrow{F} + \overrightarrow{AB} + \overrightarrow{F}^2 + \overrightarrow{F}_2 + \overrightarrow{F}_1^2 \\
         \overrightarrow{AB}^2+\frac{\overrightarrow{AB}}{\overrightarrow{AB}} + \sqrt{\overrightarrow{AB}} + \left\lvert\overrightarrow{AB}\right\rvert
     \end{array}