patch 9.0.1945: Vim9: missing support for ro-vars in interface

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>
diff --git a/src/testdir/test_vim9_disassemble.vim b/src/testdir/test_vim9_disassemble.vim
index 4cf4dfe..206908b 100644
--- a/src/testdir/test_vim9_disassemble.vim
+++ b/src/testdir/test_vim9_disassemble.vim
@@ -3052,15 +3052,15 @@
   var lines =<< trim END
     vim9script
     interface I
-      public this.o_var: number
-      public this.o_var2: number
+      this.o_var: number
+      this.o_var2: number
     endinterface
 
     class C implements I
       public static s_var: number
-      public this.o_var: number
+      this.o_var: number
       public static s_var2: number
-      public this.o_var2: number
+      this.o_var2: number
     endclass
 
     def F1(i: I)