patch 8.2.0981: Vim9: cannot compile "[var, var] = list"

Problem:    Vim9: cannot compile "[var, var] = list".
Solution:   Implement list assignment.

src/testdir/test_vim9_script.vim

@@ -223,6 +223,14 @@ def Test_assignment_default()
   assert_equal(5678, nr)
 enddef
 
+def Test_assignment_var_list()
+  let v1: string
+  let v2: string
+  [v1, v2] = ['one', 'two']
+  assert_equal('one', v1)
+  assert_equal('two', v2)
+enddef
+
 def Mess(): string
   v:foldstart = 123
   return 'xxx'