[4.x] Add children tag

[ryanmitchell]

Sometimes you want to get and loop over the children of the current page, for example to show an inline sub-nav. You can do this with the right combination of params on the {{ nav }} tag but given there is a {{ parent }} tag, people might expect an equivalent {{ children }} tag.

Closes #5567

[jasonvarga]

Neat! Need to double check that this doesn't interfere with the children variable when inside a nav tag.

[ryanmitchell]

@jasonvarga added a test for that

[duncanmcclean]

This is working great for me.