Support 3 level nesting

[arielj]

This PR adds support for creating a form with 3 levels: the parent form, dynamic children, and dynamic grandchildren.

This requires a new convention to work properly since it's simple to detect when we nave this extra nesting to do it automatically.

• If more than one nesting is used, the new idx_placeholder option must be defined by the user of the gem and each level must have a different value
• The wrapper ids for a nested element must include the parent's id and idx_placeholder value

The test app includes an example of these conventions:

https://github.com/arielj/vanilla-nested/blob/support-3-level-nesting/test/VanillaNestedTestRails7/app/views/users/_form.html.erb#L5

https://github.com/arielj/vanilla-nested/blob/support-3-level-nesting/test/VanillaNestedTestRails7/app/views/users/_pet_fields.html.erb#L12-L17