patch 9.0.1906: Vim9: Interfaces should not support class methods and variables

Problem:  Vim9: Interfaces should not support class methods and
          variables
Solution: Make sure interface follow the interface specification

Vim9 interface changes to follow the new interface specification:

1) An interface can have only read-only and read-write instance
   variables.
2) An interface can have only public instance methods.
3) An interface cannot have class variables and class methods.
4) An interface cannot have private instance variables and private
   instance methods.
5) A interface can extend another interface using "extends". The
   sub-interface gets all the variables and methods in the super
   interface.

That means:
- Interfaces should not support class methods and variables.
- Adjust error numbers and add additional tests.
- Interface methods can be defined in one of the super classes.
- Interface variables can be defined in one of the super classes.
  and instance variables can be repeated in sub interfaces.
- Check the class variable types with the type in interface.

closes: #13100

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1906,
 /**/
     1905,
 /**/