patch 8.2.4589: cannot index the g: dictionary

Problem:    Cannot index the g: dictionary.
Solution:   Recognize using "g:[key]". (closes #9969)

src/testdir/test_vim9_assign.vim

@@ -1116,6 +1116,14 @@ def Test_assignment_dict()
   END
   v9.CheckDefAndScriptSuccess(lines)
 
+  lines =<< trim END
+    var key = 'foo'
+    g:[key] = 'value'
+    assert_equal('value', g:foo)
+    unlet g:foo
+  END
+  v9.CheckDefAndScriptSuccess(lines)
+
   lines =<< trim END
     var dd = {one: 1}
     dd.one) = 2