[js] Fix type names in debugger

[kLabz]

Previous behavior

When compiling with -D js-classic and using (chrome/vscode/nwjs) javascript debugger, when $hxClasses are generated (this behavior can be forced with -dce no for example) we end up with weird type names in debugger:

This is because javascript debugger trips on this expression while generating a function name:

var foo_bar_Foo = $hxClasses["foo.bar.Foo"] = function() {

---

When also using -D js-unflatten, we get an even worse variant:

This is because javascript debugger trips on this expression while generating a function name:

foo.bar.Foo = $hxClasses["foo.bar.Foo"] = function() {

Proposed solution

In order to avoid js debugger messing up function (and so types) names, and since support for displayName has been removed, we need to clean up that function declaration and move the $hxClasses / $hx_exports to after the function declaration. Since that leads to a bit bigger output, I only enabled this behavior when in debug mode.

foo.bar.Foo = function() {
  // ...
};
$hxClasses["foo.bar.Foo"] = foo.bar.Foo;

[sebthom]

That is cool stuff. @Simn do you have any objections to merge this?

[Simn]

Looks good to me!