Refactor line number implementation

+ Add relative line number support (see doom-line-numbers-style)
+ Update doom/toggle-line-numbers
+ New hook functions: doom|enable-line-numbers,
  doom|disable-line-numbers

Addresses #156

core/packages.el

@@ -19,7 +19,8 @@
 (package! highlight-numbers)
 (unless (boundp 'display-line-numbers)
   (package! nlinum)
-  (package! nlinum-hl))
+  (package! nlinum-hl)
+  (package! nlinum-relative))
 (package! rainbow-delimiters)
 (package! vi-tilde-fringe)
 (package! visual-fill-column)