Problem: lalloc(0) error for a class without members.
Solution: Don't allocate room for members if there aren't any.
Don't create the class if there was an error.
Problem: Classes are not documented or implemented yet.
Solution: Make the first steps at documenting Vim9 objects, classes and
interfaces. Make initial choices for the syntax. Add a skeleton
implementation. Add "public" and "this" in the command table.
