Add underscroll with style class

### Problem

I've seen devs asking how they can add underscroll to a specific view like they get in popovers (see: https://github.com/hezral/quickword/issues/5); however, in the past we've been concerned about being able to reliably add it by default (due to differing view background colors).

### Proposal

We could at least add it as a style class that can be added so devs can opt into it.

### Prior Art (Optional)

Popovers have underscroll fade to the background color.