refactor(users/glittershark): Rename to grfn

Rename my //users directory and all places that refer to glittershark to
grfn, including nix references and documentation.

This may require some extra attention inside of gerrit's database after
it lands to allow me to actually push things.

Change-Id: I4728b7ec2c60024392c1c1fa6e0d4a59b3e266fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2933
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

users/grfn/emacs.d/nix.el

@@ -0,0 +1,30 @@
+;;; -*- lexical-binding: t; -*-
+
+(defun nix-buffer-type ()
+  "Returns:
+
+'home-manager, if the current buffer is a home-manager module
+'nixos, if the current buffer is a nixos module
+nil, if none of the above are the case"
+  (when buffer-file-name
+    (pcase buffer-file-name
+      ((rx (0+ nonl) "system/home" (0+ nonl) ".nix" eos)
+       'home-manager)
+      ((rx (0+ nonl) "system/system" (0+ nonl) ".nix" eos)
+       'nixos))))
+
+(defun set-nix-compile-command ()
+  "Set the compile command for the current buffer based on the type of nix
+buffer it is, per `nix-buffer-type'"
+  (interactive)
+  (when-let ((btype (nix-buffer-type)))
+    (setq-local
+     compile-command
+     (case btype
+       ('home-manager "home-manager switch")
+       ('nixos "sudo nixos-rebuild switch")))))
+
+(add-hook 'nix-mode-hook #'set-nix-compile-command)
+
+(map! (:map nix-mode-map
+       (:n "g SPC" #'compile)))