patch 9.0.1909: Vim9: problem calling class method from other class

patch 9.0.1909: Vim9: problem calling class method from other class

Problem:  Vim9: problem calling class method from other class
Solution: Fix this problem, fix readonly object access, update error
          messages.

Calling a class method from another method without the class name prefix
doesn't work properly.

A readonly object variable is modifiable outside the class using a
nested object assignment.

Remove the unused E1338 error message.

Update error messages.

closes: #13116

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