[js] shallow-expose writes to global

According to the doc, `-D shallow-expose` should not write to the global window / export.

https://haxe.org/manual/target-javascript-expose.html#shallow-expose

3.2.1 generates the same code as that in the example.

But since 3.3.0 it writes to global, with `$hx_exports` declared as:

```js
var $hx_exports = typeof exports != "undefined" ? exports : typeof window != "undefined" ? window : typeof self != "undefined" ? self : this;
```