[C#] Support nullable reference types. A generic C# option to the
%module directive allows one to add in any code at the beginning of every
C# file. This can add the #nullable enable preprocessor directive at the beginning
of every C# file in order to enable nullable reference types as follows:
%module(csbegin="#nullable enable\n") mymodule
Closes#2681
[D, Java] Add the dbegin option to the %module directive for generating code at
the beginning of every D file. Similarly javabegin for Java. This enables one
to add a common comment at the start of each D/Java file.
William S Fulton