[js][es6] add ES6 module exports

[zhaoterryy]

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.

@:expose
class Test {
    function new() {
    }
}

would produce

export class Test {
    constructor() {
    }
}

[back2dos]

Related: It would also be good if there were a @:jsRequire equivalent for es6 import.

[impaler]

One advantage to the javascript target having support for es6 modules is the "tree-shaking" feature most popular js blunders are now providing. So they can include only the modules that are being used, it doesn't seem possible for these blunders to optimize a Haxe compiled js project without it.

• https://webpack.js.org/guides/tree-shaking
• https://rollupjs.org/guide/en#tree-shaking