Problem: Vim9: missing support for ro-vars in interface
Solution: Support only read-only object variables in an interface,
add additional checks when parsing class definitions.
closes: #13183
cloess: #13184
cloess: #13185.
closes: #13188
Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>