💥 Rewrite line numbers system for Emacs 25

This removes the various doom-line-number* variables and replaces it
with the Emacs 26 display-line-numbers API, which I've ported to Emacs
25.x (however, it uses nlinum under the hood, and not all of
display-line-numbers options are supported).

modules/app/write/config.el

@@ -7,9 +7,7 @@
   "What to set `line-spacing' in `+write-mode'.")
 
 (defun +write|init-line-numbers ()
-  (if +write-mode
-      (doom|enable-line-numbers)
-    (doom|disable-line-numbers)))
+  (display-line-numbers-mode (if +write-mode +1 -1)))
 
 (defun +write|init-mixed-pitch ()
   (mixed-pitch-mode (if +write-mode +1 -1)))