Add option to mark generated classes as internal

[wilkomanger]

Now the classes are by default marked as public, it would be nice for libraries using this package to have these classes marked as internal so they won't be exported.

[lppedd]

This request should probably go into the main ANTLR repo, as that's where the StringTemplate template data is generated.

cc @ftomassetti, I'd close this one as we can't do it.

[ftomassetti]

If I understand correctly, we would need a change in the ANTLR tool to have the possibility to selectively mark some classes as internal. We could however mark all the generated classes as internal (or not), based on some flag