Turn source code into a plain ES module without UMD wrapper
Turn source code into a plain ES module without UMD wrapper

As part of transition the source code to be a proper plain ES module,
we're removing the handcrafted UMD wrapper around the source code.

This means the UMD wrapper will have to come from elsewhere; a build
step introduced later.

janl/mustache.js,janl/mustache.js,phillipj/mustache.js,janl/mustache.js,janl/mustache.js,phillipj/mustache.js,phillipj/mustache.js,phillipj/mustache.js,phillipj/mustache.js,janl/mustache.js