checkers/spell: replace flyspell with spell-fu

Spell-fu is significantly faster, but does produce more false
positives (more faces must be added to spell-fu-faces-exclude to reduce
these).

Unfortunately, there is no fancy "correction" interface for spell-fu
yet, so we'll have to resort to ispell-word (on z=) for now.

modules/config/default/+emacs-bindings.el

@@ -280,7 +280,7 @@
         :desc "org-tree-slide mode"        "p" #'org-tree-slide-mode)
        :desc "Read-only mode"               "r" #'read-only-mode
        (:when (featurep! :checkers spell)
-        :desc "Flyspell"                   "s" #'flyspell-mode)
+        :desc "Spell checker"              "s" #'spell-fu-mode)
        (:when (featurep! :lang org +pomodoro)
         :desc "Pomodoro timer"             "t" #'org-pomodoro)
        (:when (featurep! :ui zen)