Create design tokens for `ScrollView` shadows

[hubacekj]

We want to be able to control ScrollView's startShadowBackground and endShadowBackground in theme.
Does @adamkudrna want to further specify the task?

[adamkudrna]

Comes after #423.

[adamkudrna]

It could be something like ScrollView__start-shadow__background and ScrollView__end-shadow__background.