patch 8.2.1447: Vim9: return type of keys() is list<any>

Problem:    Vim9: return type of keys() is list<any>.
Solution:   Should be list<string>. (closes #6711)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 3e72120..75db260 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1283,6 +1283,11 @@
   assert_equal(6, res)
 enddef
 
+def Test_keys_return_type()
+  const var: list<string> = #{a: 1, b: 2}->keys()
+  assert_equal(['a', 'b'], var)
+enddef
+
 def Test_reverse_return_type()
   let l = reverse([1, 2, 3])
   let res = 0