[js][es6] add ES6 module exports

Add a way to export classes and functions as an ES6 module. Maybe overriding the ```@:expose``` behavior when ```-D js_es=6```? Or introducing a new metadata?

i.e.
```hx
@:expose
class Test {
function new() {
}
}
```
would produce
```js
export class Test {
constructor() {
}
}
```