fix: whitespace has a separate built-in package

It has a better hook (whitespace-cleanup) than the barebones delete-trailing-whitespace.

config.org

@@ -72,10 +72,16 @@ The audible bell is annoying AF.
 (setq column-number-mode 1)
 #+END_SRC
 
-** Delete trailing whitespaces
+** Whitespace cleanup
 
 #+BEGIN_SRC emacs-lisp
-(add-hook 'before-save-hook 'delete-trailing-whitespace)
+(use-package whitespace
+  :ensure nil
+  :defer t
+  :hook (before-save-hook . whitespace-cleanup)
+  ;; if we wanna remove this hook at any time, eval:
+  ;; (remove-hook 'before-save-hook #'whitespace-cleanup)
+  )
 #+END_SRC
 
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