checkers/spell: add +flyspell flag

spell-fu lacks support for multiple dictionaries, affix expansion, and
many non-English dictionaries, so I've added back flyspell support to
the spell module, but opt-in, because it is still the significantly
slower option and spell-fu may support them one day.

If not, tlikonen/wcheck-mode is another alternative to consider.

Fixes #3813

modules/checkers/spell/autoload/flyspell.el

@@ -0,0 +1,28 @@
+;;; checkers/spell/autoload/flyspell.el -*- lexical-binding: t; -*-
+;;;###if (featurep! +flyspell)
+
+;;;###autodef
+(defalias 'flyspell-mode! #'flyspell-mode)
+
+(defvar +spell--flyspell-predicate-alist nil
+  "TODO")
+
+;;;###autodef
+(defun set-flyspell-predicate! (modes predicate)
+  "TODO"
+  (declare (indent defun))
+  (dolist (mode (doom-enlist modes) +spell--flyspell-predicate-alist)
+    (add-to-list '+spell--flyspell-predicate-alist (cons mode predicate))))
+
+;;;###autoload
+(defun +spell-init-flyspell-predicate-h ()
+  "TODO"
+  (when-let (pred (assq major-mode +spell--flyspell-predicate-alist))
+    (setq-local flyspell-generic-check-word-predicate (cdr pred))))
+
+;;;###autoload
+(defun +spell-correction-at-point-p (&optional point)
+  "TODO"
+  (cl-loop for ov in (overlays-at (or point (point)))
+           if (overlay-get ov 'flyspell-overlay)
+           return t))