Configuration to enable scrollbar rendering for scrollable `Element`s.
Currently only the fg `Color` of the scrollbar can be configured while
the background uses the same fg `Color` but adds the emphasis `.dim` to
make it obvious what the is the actual scrollbar. In the future it might
be necessary to provide the user with more options to configure the
representation of the scrollbar.
Tests have been added to test the scrollbar rendering and placement
accordingly.
