Make sure \surd is symmetric vertically. (mathjax/MathJax#3468)

Author: dpvc

testsuite/tests/input/tex/Base.test.ts

@@ -7928,7 +7928,7 @@ describe('Mathchar0mo', () => {
     toXmlMatch(
       tex2mml('\\surd'),
       `<math xmlns="http://www.w3.org/1998/Math/MathML" data-latex="\\surd" display="block">
-         <mo stretchy="false" data-latex="\\surd">&#x221A;</mo>
+         <mo symmetric="true" stretchy="false" data-latex="\\surd">&#x221A;</mo>
        </math>`
     );
   });

ts/input/tex/base/BaseMappings.ts

@@ -169,7 +169,7 @@ new sm.CharacterMap('mathchar0mi', ParseMethods.mathchar0mi, {
  * Macros for operators.
  */
 new sm.CharacterMap('mathchar0mo', ParseMethods.mathchar0mo, {
-  surd: '\u221A',
+  surd: ['\u221A', { symmetric: true }],
 
   // big ops
   coprod: ['\u2210', { movesupsub: true }],