Replace delete-trailing-whitespace with ws-butler

ws-butler only strips trailing spaces on lines that have been modified.

+ ws-butler is disabled if editorconfig enables
  trim_trailing_whitespace, which resorts to delete-trailing-whitespace
  instead.
+ Updates doom|(enable|disable)-delete-trailing-whitespace hooks to use
  ws-butler-mode.

core/packages.el

@@ -32,6 +32,7 @@
 (package! pcre2el)
 (package! smartparens)
 (package! undo-tree)
+(package! ws-butler)
 
 ;; core-projects.el
 (package! projectile)