fix navigateNext to start at top of next vertical stack

[gchriz]

Some navigation issue regarding remembered vertical positions drove me crazy until I found out some details now.

After navigating left/right between vertical stacks below the top position and then continuing the presentation with Next (space or N) the presentation jumps from the bottom of current stack to the remembered position of the next stack to the right. And NOT to the top position as expected in the normal flow.
Navigating backwards (shift-space or P) is working correctly: The bottom slide of the next stack to the left is jumped to.

As far as I understand it the exact same behaviour is described in the older issue #2838.

So I fixed it in this PR.

(And I learned about npm etc. so that I was able to build reveal.js' "dist" directory.)

[gchriz]

Probably the same fix needs to be applied one "else if" block above for config.rtl and so replacing the simple navigateLeft() there.
Since I don't know anything about RTL I didn't change that yet. But of course can do it later.