[js] don't generate constructor function for KAbstractImpl

Despite being classes under the hood, abstract implementations is just a collections of static methods and can't be constructed, so generating constructor function in JS doesn't make sense.

We did this cleanup for C# (now KAbstractImpl are just sealed classes with nothing but static methods) and for JS I think they should be generated as simple objects for the sake of cleanliness of generated JS. It also makes the distinction between regular classes and abstract implementations more clear to a person reading the generated code (most of people do that when they start with haxe).

This may or may not (probably not) have some positive impact on performance, depending on how modern js engines work with regard to an object being a constructor function or simple hash.

Before:
```js
_Main.A_Impl_ = function() { };
_Main.A_Impl_.doStuff = function(this1) {
```

After:
```js
_Main.A_Impl_ = { };
_Main.A_Impl_.doStuff = function(this1) {
```

I thought about generating it even simplier, like this:

```js
_Main.A_Impl_ = {
    doStuff: function(this1) {
    }
};
```

But then it's not clear how to deal with __init__ that is supposed to run before static var initialization.