Closes https://github.com/janl/mustache.js/pull/738

In the spirit of keeping the public API of mustache.js as small as
possible for maintainence reasons, the undocumented and un-tested
`.to_html()` method is removed.

The functionality involved, where it can rather invoke an optional
function provided with the result of `.render()`, instead of returning
it as a string like `.render()` does, is something that using projects
very easily can do themselfs -- it does not have to be provided by
mustache.js.

These changes allows the internal template cache to
be customised, either by disabling it complete or
providing a custom implementation of how templates
are cached.

Minor adjustments needed to make the TypeScript compiler
that is built into Deno, be happy with how mustache.js'
ES module source looks in terms of function parameters
passed and object mutability.

Refs https://github.com/phillipj/mustache.js/pull/1

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.

Added `-v` option to CLI, bugfix for rendering `Number` when it was the
context, and specified files in package.json for cleaner `npm install`.