Merge pull request #1174 from mathjax/mhchem-stretchies

Add stretchy characters needed by mhchem

Author: dpvc

components/mjs/input/tex/extensions/mhchem/mhchem.js

@@ -1 +1,4 @@
 import './lib/mhchem.js';
+import {fontExtension} from '../../extension.js';
+
+fontExtension('[tex]/mhchem', 'mathjax-mhchem-font-extension');