Renamed mustache.js -> mustache.mjs to transition to being an ES module

In an effort of moving with recent changes in Node.js and its built-in
support for ES modules, we're making the main source code reside inside
`mustache.mjs`. This will also allow Deno users to use this package as
an ES module directly by importing `mustache.mjs`.

An ES3 / UMD version of the source code is still planned to exist in
`mustache.js`, but will involve a build step to transform the ES module
source into a plain .js version.

By renaming the file *before* changing any contents, git will
recognise the file has been renamed and therefore allows us to see all
historical changes to the old .js source code with the `--follow` argument:

```
$ git log --follow -- mustache.mjs
```

Without ensuring git sees this is a file rename, we will in practise
loose the old source code's history, or at least make it quite weird
and less intuitive to find. That's not fair to the historical
contributors and in general getting hold of a well documented log of
changes to a file in git, is extremely valuable in those few scenarios
where it's *really* needed.