patch 8.2.2798: Vim9: redir to variable with append does not accept an index

Problem:    Vim9: redir to variable with append does not accept an index.
Solution:   Make the appending work.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index ddf3df2..ab2ad67 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1230,6 +1230,11 @@
   redir END
   assert_equal({l: ["\ndict-list"]}, dl)
 
+  redir =>> d.redir
+    echo 'more'
+  redir END
+  assert_equal({redir: "\ndict\nmore"}, d)
+
   var lines =<< trim END
     redir => notexist
   END