feat(ngRepeat): add $prev and $next

[nswbmw]

In some cases, i need to get previous element before this $index or get next element after this $index. Array works well, but Object not. Maybe others need this feature too.

[caitp]

So, I can understand why you'd want this feature, but we do see a real performance impact from adding properties to each scope, it's not ideal. I think we need to be very careful about adding new ones

[gkalpak]

Just adding a little context:

There's been a couple of similar PRs already (e.g. #9795, #10362).

IIRC, it was discussed during the planning meeting for 1.4.
IIRC again, people (@IgorMinar ?) weren't too keen on adding new properties on all ngRepeat generated scopes, but there were some interesting alternatives discussed (e.g. adding them on-demand, having a getter property that caclulates them only if they are requested (by @lgalfaso ?)).

I am not sure what the final verdict was (some memory, huh ?) 😃

[Narretz]

I think the consensus was to add the extra properties on-demand. But we didn't really work on it.