Add underscroll with style class

[cassidyjames]

Problem

I've seen devs asking how they can add underscroll to a specific view like they get in popovers (see: hezral/quickword#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.

[alice-mkh]

FWIW I'm doing this in libadwaita as well (not landed yet tho), and I ended up with the following: