patch 8.2.1359: Vim9: cannot assign to / register in Vim9 script

Problem:    Vim9: cannot assign to / register in Vim9 script.
Solution:   Adjust check for assignment in Vim9 script. (closes #6567)

src/testdir/test_vim9_script.vim

@@ -423,6 +423,23 @@ def Test_assignment_vim9script()
     let ll =
           Func()
     assert_equal([1, 2], ll)
+
+    @/ = 'text'
+    assert_equal('text', @/)
+    @0 = 'zero'
+    assert_equal('zero', @0)
+    @1 = 'one'
+    assert_equal('one', @1)
+    @9 = 'nine'
+    assert_equal('nine', @9)
+    @- = 'minus'
+    assert_equal('minus', @-)
+    if has('clipboard_working')
+      @* = 'star'
+      assert_equal('star', @*)
+      @+ = 'plus'
+      assert_equal('plus', @+)
+    endif
   END
   CheckScriptSuccess(lines)
 enddef