Make frown/smile stretch via an alias for top/bottom parenthesis [was: Add permanent workaround for mover with frown / arc]

None of the supported fonts can do a stretchy arc (using ⌢) but we can do stretchy parenthesis.

Something like

<script type="text/x-mathjax-config"> 
    MathJax.Hub.Register.StartupHook("MathML Jax Ready",function () { 
      MathJax.InputJax.MathML.prefilterHooks.Add(function (data) { 
        data.math = data.math.replace(/⌢/g, '&#x23dc;'); 
      }); 
    }); 
</script> 

is a decent hack (better on some fonts like STIX).

This feature request is to add this internally.