patch 8.2.3127: Vim9: no error when adding number to list of string

Problem:    Vim9: no error when adding number to list of string.
Solution:   Check the value type. (closes #8529)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index f6b5a09..8a6af57 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -104,6 +104,13 @@
       add(l, 123)
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var l: list<string> = ['a']
+      l->add(123)
+  END
+  CheckScriptFailure(lines, 'E1012: Type mismatch; expected string but got number', 3)
 enddef
 
 def Test_add_blob()