Support dtl loop over objects assigning key/value pairs.

Refer [#17251](https://bugs.dojotoolkit.org/ticket/17251)

In summary, the for loop tag over objects drops the key, so syntax like:

```{% for key, value in object %}```

Doesn't work as expected.

This patch adjusts the `ForNode` so that objects are assigned as key/value pairs to the assignment arguments.