emacs-configuration: Add nix-mode

* user-config/emacs/.emacs.d/config.org (Nix): Add section on nix, and setup nix-mode

* .guix/rekahsoft/guix-config/home.scm (%emacs-manifest): Install emacs-nix-mode

.guix/rekahsoft/guix-config/home.scm

@@ -118,6 +118,7 @@
        "emacs-mu4e-conversation"
        "emacs-multi-term"
        "emacs-multiple-cursors"
+       "emacs-nix-mode"
        "emacs-no-littering"
        "emacs-ob-async"
        "emacs-org"

user-config/emacs/.emacs.d/config.org

@@ -2398,6 +2398,13 @@ Use [[https://github.com/emacs-dashboard/emacs-dashboard][emacs-dashboard]].
   (use-package rust-mode)
 #+end_src
 
+** Nix
+
+#+begin_src emacs-lisp
+  (use-package nix-mode
+    :mode "\\.nix\\'")
+#+end_src
+
 ** Lispy
 
 #+begin_src emacs-lisp