patch 8.2.3051: Vim9: for loop with one list variable does not work

Problem:    Vim9: for loop with one list variable does not work.
Solution:   Use a separate flag for unpacking a list. (closes #8452)

src/testdir/test_vim9_script.vim

@@ -2385,6 +2385,13 @@ def Test_for_loop()
       endfor
       assert_equal('1a2b', res)
 
+      # unpack with one var
+      var reslist = []
+      for [x] in [['aaa'], ['bbb']]
+        reslist->add(x)
+      endfor
+      assert_equal(['aaa', 'bbb'], reslist)
+
       # loop over string
       res = ''
       for c in 'aéc̀d'