patch 9.0.1948: Vim9: object variable "this." should only be used in constructor

Problem:  Vim9: object variable "this." should only be used in
          constructor
Solution: Disallow to this in normal object methods (other than
          constructors)

closes: #13152
closes: #13212

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: h-east <h.east.727@gmail.com>

src/testdir/test_vim9_class.vim

@@ -955,6 +955,17 @@ def Test_class_new_with_object_member()
   END
   v9.CheckSourceFailure(lines, 'E1013:')
 
+  lines =<< trim END
+    vim9script
+
+    class C
+      this.str: string
+      def MethodA(this.str)
+      enddef
+    endclass
+  END
+  v9.CheckSourceFailure(lines, 'E1390: Cannot use an object variable "this.str" except with the "new" method')
+
   lines =<< trim END
       vim9script