Add toggle for scroll to top in customizer

[timelsass]

A toggle to turn off/on the scroll to top arrow functionality in the WordPress customizer would be a nice feature addition.

I think an appropriate place to add this in the customizer would be: "Design > Site Content > Scroll To Top"

This lib is separated out into configs already, so additional controls to handle the many options it has shouldn't be hard to implement. I think for the time being the toggle wouldn't require any adjustments to the library that handles the functionality, but in the future I'd like to add some icon selection, alignment, and color selection options and think we would rewrite this library to use our own implementation and get rid of the third-party dep.