Support \mkern as an alias for \kern.

src/functions.js

@@ -205,7 +205,9 @@ defineFunction("\\rule", {
     };
 });
 
-defineFunction("\\kern", {
+// TODO: In TeX, \mkern only accepts mu-units, and \kern does not accept
+// mu-units. In current KaTeX we relax this; both commands accept any unit.
+defineFunction(["\\kern", "\\mkern"], {
     numArgs: 1,
     argTypes: ["size"],
 }, function(context, args) {