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/config.el

@@ -0,0 +1,8 @@
+;;; ui/smooth-scroll/config.el -*- lexical-binding: t; -*-
+
+(use-package! ultra-scroll
+  :hook (doom-first-input . ultra-scroll-mode)
+  :hook (doom-first-file . ultra-scroll-mode)
+  :init
+  (setq scroll-conservatively 101
+        scroll-margin 0))