macro functions in externs default to private

As discovered in https://github.com/HaxeFoundation/haxe/pull/9205, the default visibility of `macro` functions in externs seems to be `private`, which seems very counter-intuitive. That creates the following situation (visibility that Haxe uses by default in comments, you can also easily see this by hovering over the function names in VSCode):

```haxe
extern class Test {
/* public */ function a():Void;
/* public */ inline function b():Void {}
/* private */ macro inline function c():Void {}
}
```

Surely this is just an oversight / not an intentional design decision?