patch 8.2.2771: Vim9: assignment not recognized if declaration was skipped

Problem:    Vim9: assignment not recognized if declaration was skipped.
Solution:   Also recognized an assignment if the variable does not exist.
            (closes #8108)

src/testdir/test_vim9_assign.vim

@@ -249,6 +249,18 @@ def Test_assignment()
   END
 enddef
 
+def Test_skipped_assignment()
+  var lines =<< trim END
+      for x in []
+        var i: number = 1
+        while false
+          i += 1
+        endwhile
+      endfor
+  END
+  CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_assign_unpack()
   var lines =<< trim END
     var v1: number