patch 8.2.4360: Vim9: allowing use of "s:" leads to inconsistencies

Problem:    Vim9: allowing use of "s:" leads to inconsistencies.
Solution:   Disallow using "s:" in Vim9 script at the script level.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index a9309be..e7cdcc3 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1718,7 +1718,12 @@
   lines =<< trim END
       s:notexist:repl
   END
-  v9.CheckDefAndScriptFailure(lines, ['E488: Trailing characters: :repl', 'E121: Undefined variable: s:notexist'], 1)
+  v9.CheckDefAndScriptFailure(lines, ['E488: Trailing characters: :repl', 'E1268:'], 1)
+
+  lines =<< trim END
+      notexist:repl
+  END
+  v9.CheckDefAndScriptFailure(lines, ['E476:', 'E492:'], 1)
 
   lines =<< trim END
       s-pat-repl