fix(wpcarro/emacs): Support OSX build of Emacs

**TL;DR:**
Most of these changes predicate behavior on the platform:
- At buildtime this is `localSystem == $something`. (`localSystem` is from
  `readTree`)
- At runtime this is `(memq window-system '(ns))`.
- Add `coreutils` so `dired` listing with `--group-directories-first` works
  because that flag depends on the GNU version of `ls`.

**Background:**
I need to support a bunch of OSX users at $WORK. As such, I'm planning
on using my MBP for the next few weeks to build empathy for our
userbase and polish some currently rough edges.

If I'm going to get an serious work done, I need my Emacs setup. Step
one is making sure it can build and run.

Change-Id: I918efccfa5f149e218aeea476c2c7df1c7b64ae8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7309
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/emacs/.emacs.d/wpc/wpc-ui.el

@@ -164,8 +164,12 @@
 (colorscheme-whitelist-set 'doom-peacock)
 
 (when window-system
-  (let ((font "Monospace"))
-    (fonts-whitelist-set font)
+  ;; On OSX, JetBrainsMono is installed as "JetBrains Mono", and I'm
+  ;; not sure how to change that.
+  (let ((font (if (memq window-system '(ns))
+		  "JetBrains Mono"
+		"JetBrainsMono")))
+    (fonts-set font)
     ;; Some themes (e.g. doom-acario-*) change the font for comments. This
     ;; should prevent that.
     (set-face-attribute font-lock-comment-face nil