patch 9.0.2167: Vim9: not consistently using :var for declarations

Problem:  Vim9-script object/class variable declarations use syntax
	  that is inconsistent with the rest of the language.
Solution: Use :var to declare object and class variables.

closes: #13670

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 216118e..1267936 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -4845,7 +4845,7 @@
       vim9script
 
       class Foo
-        this.val: number
+        var val: number
         def Add()
           echo this.val
         enddef