Make sure \require{mhchem} doesn't override [Contrib]/mhchem

If the new [Contrib]/mhchem/mhchem.js is already loaded, then \require{mhchem} still loads the current [MathJax]/extensions/TeX/mhchem.js file, but it shouldn't, as the old version will replace the new one in this case.

The old file should check if mhchem is already loaded and not override it if it is.