st/scroll-view-fade: Add :extend-fade-area property

st/scroll-view-fade: Add :extend-fade-area property

Since commit f60a469a34e, everything outside the fade area is painted
fully transparent. That is required for the fade effect during app
grid navigation, to prevent unfaded parts of surrounding pages
becoming visible on wide-screen displays.

However in most other cases, that behavior is the exact opposite
of what we want: Elements outside the fade area (like scroll bars)
should never fade.

In order to fix the regular case, hide the new behavior behind a
property.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4234

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1831>