patch 8.2.3053: Vim9: cannot assign to @@ in :def function

Problem:    Vim9: cannot assign to @@ in :def function
Solution:   Handle '@' like '"'.

src/testdir/test_vim9_assign.vim

@@ -228,16 +228,6 @@ def Test_assignment()
   CheckDefFailure(['$SOME_ENV_VAR += "more"'], 'E1051:')
   CheckDefFailure(['$SOME_ENV_VAR += 123'], 'E1012:')
 
-  lines =<< trim END
-    @c = 'areg'
-    @c ..= 'add'
-    assert_equal('aregadd', @c)
-  END
-  CheckDefAndScriptSuccess(lines)
-
-  CheckDefFailure(['@a += "more"'], 'E1051:')
-  CheckDefFailure(['@a += 123'], 'E1012:')
-
   v:errmsg = 'none'
   v:errmsg ..= 'again'
   assert_equal('noneagain', v:errmsg)
@@ -249,6 +239,21 @@ def Test_assignment()
   END
 enddef
 
+def Test_assign_register()
+  var lines =<< trim END
+    @c = 'areg'
+    @c ..= 'add'
+    assert_equal('aregadd', @c)
+
+    @@ = 'some text'
+    assert_equal('some text', getreg('"'))
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  CheckDefFailure(['@a += "more"'], 'E1051:')
+  CheckDefFailure(['@a += 123'], 'E1012:')
+enddef
+
 def Test_reserved_name()
   for name in ['true', 'false', 'null']
     CheckDefExecAndScriptFailure(['var ' .. name .. ' =  0'], 'E1034:')