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
```

Also change linting w/eslint to target the `.mjs` file, since the output
of rollup that ends up in the old `mustache.js` file is not sensible to
do linting on.

.eslintrc

@@ -16,5 +16,14 @@
     "no-use-before-define": 0,
     "no-process-exit": 0,
     "strict": 0
-  }
+  },
+  "overrides": [
+    {
+      "files": ["mustache.mjs"],
+      "parserOptions": {
+        "sourceType": "module",
+        "ecmaVersion": 2015
+      }
+    }
+  ]
 }