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

modules/ui/smooth-scroll/packages.el

@@ -0,0 +1,6 @@
+;; -*- no-byte-compile: t; -*-
+;;; ui/smooth-scroll/packages.el
+
+(package! ultra-scroll
+  :recipe (:host github :repo "jdtsmith/ultra-scroll")
+  :pin "b72c507f6702db18d971a6b6bdc692e260f21159")