feat(emacs): add support for just-mode

- Added `just-mode` to the Emacs package list in Nix configuration
- Configured `just-mode` in Emacs init file with autoload and file
  associations for Justfile syntax
- Included `rustup` in the advanced Nix package list

Author: takeokunn

home-manager/packages/advanced.nix

@@ -1,6 +1,8 @@
 { pkgs, nodePkgs }:
 with pkgs;
 [
+  rustup
+
   # lint
   similarity
 

home-manager/packages/emacs/epkgs/packages/language/default.nix

@@ -49,6 +49,7 @@ with epkgs;
   js2-mode
   json-mode
   jsonnet-mode
+  just-mode
   kotlin-mode
   lua-mode
   markdown-mode

home-manager/programs/emacs/elisp/init.org

@@ -711,6 +711,13 @@
     (define-key jsonnet-mode-map (kbd "C-c C-f") #'jsonnet-jump)
     (define-key jsonnet-mode-map (kbd "C-c C-r") #'jsonnet-reformat-buffer))
 #+end_src
+** just-mode
+#+begin_src emacs-lisp :tangle yes
+  (autoload-if-found '(just-mode) "just-mode" nil t)
+
+  (add-to-list 'auto-mode-alist '("/[Jj]ustfile\\'" . just-mode))
+  (add-to-list 'auto-mode-alist '("\\.[Jj]ust\\(file\\)?\\'" . just-mode))
+#+end_src
 ** kotlin-mode
 #+begin_src emacs-lisp :tangle yes
   (autoload-if-found '(kotlin-mode) "kotlin-mode" nil t)