Generated classes Access Modifiers for C#

I think the topic is recurrent, but nobody has done anything for it in the original ANTLR project, but rather some hacks on local checkouts.
I would like to have the possibility to restrict the visibility of the generated C# Lexer/Parser classes (and related ones, such as Visitors): I presume this should be an easy change, and I see the Swift generator has been modified quickly.

Is it there any guideline on the possibility to extend the options of the files, so that I can make the change myself in the CSharp.std and send the pull request?

Start by reading the CSharp.std generator file and the referenced Swift generator changes. Determine how generated C# Lexer/Parser classes and related visitors could accept visibility options; done when those generated classes can use restricted access modifiers without affecting other targets.

Written by the indexing model from the issue text.