Infinite Scroll: Update to not require jQuery

[josephscott]

When enabling the Infinite Scroll feature in Jetpack it brings with it jQuery. That is a significant amount of additional JavaScript on a page that wasn't loading jQuery before.

Describe the solution you'd like

I'd like to see an updated version of the Infinite Scroll code that did not require jQuery - a.k.a vanilla JavaScript.

Additional context

I recently ran tests under slow mobile conditions and just enabling the Infinite Scroll feature increased the start render time by 26%. One of the reasons for that is the 647% increase in the amount of JavaScript being loaded. Of that jQuery is a large chunk, at 94 KB.