module: add :ui smooth-scroll

Normally, this would be too small to warrant its own module, but smooth
scrolling is one of those things that can tip large swaths of
fence-sitting beginners into Emacs' camp. I can't help but imagine how
many people get their impression that Emacs is "laggy" or "slow" solely
from the way it scrolls.

Don't get me wrong, there *are* good reasons to call Emacs slow, but
startup and scrolling speed are not it, yet they are easily overblown
because it's the first thing you see. I might even consider enabling
this by default, but I'll make that decision later.

Fix: #8335

static/init.example.el

@@ -43,6 +43,7 @@
        ;;neotree           ; a project drawer, like NERDTree for vim
        ophints           ; highlight the region an operation acts on
        (popup +defaults)   ; tame sudden yet inevitable temporary windows
+       ;;smooth-scroll     ; So smooth you won't believe it's not butter
        ;;tabs              ; a tab bar for Emacs
        ;;treemacs          ; a project drawer, like neotree but cooler
        ;;unicode           ; extended unicode support for various languages