Introduce build step to generate .js | .min.js from .mjs

By making the main source code an ES module placed in `mustache.mjs`,
what used to be in `mustache.js` is now built based off of the ES module
source inside `mustache.mjs`.

This is done primarily to avoid breaking existing projects already using
mustache.js, by keeping the UMD-version in `mustache.js` part of the git
repository in addition to the minified version in `mustache.min.js`.

After experiment with several compilers;

- Babel
- TypeScript
- Rollup

and examining their build output, [Rollup](https://rollupjs.org/) was chosen
because of the UMD output it creates which closely resembles what we've
historically had in the previous `mustache.js` source code.

The contents of `.js | .min.js` files has been generated by running
the following npm script:

```
$ npm run build
```