Provide context to the recursive children in the structure tag

[duncanmcclean]

This pull request fixes #2609 by providing context when using recursive children inside the nav/structure tag.

Currently, inside of recursive children, you're not able to see the data for the global page. In the below example, you wouldn't be able to access {{ current_url }} in any of the children.

{{ nav:collection:pages }}
    <li>current_url: {{ current_url or 'nada' }}</li>

    {{ if children }}
        {{ *recursive children* }}
    {{ /if }}
{{ /nav:collection:pages }}

The above example is something this PR fixes. It does this by providing the context available to the tag and merging it with the entry & structure data for the child.

[jasonvarga]

Thanks!

[jasonvarga]

FYI the context should have been added to start of the array merge.
This PR introduced a bug where variables inside the page were being overridden by variables named the same in the context.

e.g. if you're on the home page that has a title of Home, then all the nav item's {{ title }} would output Home.

Fixed in a8f3f47

[duncanmcclean]

Oh.... I actually put it at the end to prevent that but thinking of it now, it should have been the other way round. 🤦‍♂️ sorry