feat(editor): improve whitespace-mode setup and cleanup on save

Move away from the whitespace-cleanup-mode package, and instead simply
define our own whitespace-cleanup-on-save-mode minor-mode that uses a
before-save-hook.

modules/editor/siren-whitespace.el

@@ -0,0 +1,41 @@
+;;; siren-whitespace.el --- jimeh's Emacs Siren: whitespace configuration.
+
+;;; Commentary:
+
+;; Basic configuration for whitespace package
+
+;;; Code:
+
+(use-package whitespace
+  :straight (:type built-in)
+  :diminish whitespace-mode global-whitespace-mode
+  :hook
+  (prog-mode . whitespace-cleanup-on-save-mode)
+
+  :preface
+  (define-minor-mode whitespace-cleanup-on-save-mode
+    "Run whitespace-cleanup on buffer save"
+    :global nil
+    (if whitespace-cleanup-on-save-mode
+        (add-hook 'before-save-hook 'whitespace-cleanup-on-save-cleanup)
+      (remove-hook 'before-save-hook 'whitespace-cleanup-on-save-cleanup)))
+
+  (defun whitespace-cleanup-on-save-cleanup()
+    (when (bound-and-true-p whitespace-cleanup-on-save-mode)
+      (whitespace-cleanup)))
+
+  :custom
+  (whitespace-line-column 80)
+  (whitespace-style '(face tabs trailing space-before-tab newline
+                           indentation space-after-tab tab-mark newline-mark
+                           empty))
+  (whitespace-display-mappings '((newline-mark ?\n
+                                               [?\u00AC ?\n] ;; \u00AC = ¬
+                                               [?$ ?\n])
+                                 (tab-mark ?\t
+                                           [?\u2192 ?\t] ;; \u2192 = →
+                                           [?\u00BB ?\t] ;; \u00BB = »
+                                           [?\\ ?\t]))))
+
+(provide 'siren-whitespace)
+;;; siren-whitespace.el ends here