[5.x] Support as on nav tag

[ryanmitchell]

This PR adds support for as as a parameter on nav tags, so you can do things like

{{ nav:main as="navnav" }} 
    {{ navnav | chunk(3) }}
        chunk
    {{ /navnav }}
{{ /nav:main }}

[jasonvarga]

Recursion doesn't work with this though. 🤔 I'm talking about this sort of stuff:

<ul>
{{ nav:main }}
    <li>
        {{ title }}
        {{ if children }}
            <ul>{{ *recursive children* }}</ul>
        {{ /if }}
    </li>
{{ /nav:main }}
</ul>

[ryanmitchell]

Yeah you lose that ability when you 'as' it... I can live with that?

[jasonvarga]

Me too I think. We could always fix it later. We'll just document the caveat.

[edalzell]

does it support navnavnavnav?

[ryanmitchell]

I'd prefer nanananananananav