Improve flyspell support

+ Make flyspell more selective about what to scan in markdown buffers.
+ Don't mark duplicates when proseline or langtool are present.
+ Add set-flyspell-predicate! for seeing mode-specific word predicates.

modules/tools/flyspell/autoload.el

@@ -0,0 +1,16 @@
+;;; tools/flyspell/autoload.el -*- lexical-binding: t; -*-
+
+(defvar +flyspell--predicate-alist nil
+  "TODO")
+
+;;;###autodef
+(defun set-flyspell-predicate! (modes predicate)
+  "TODO"
+  (dolist (mode (doom-enlist modes) +flyspell--predicate-alist)
+    (add-to-list '+flyspell--predicate-alist (cons mode predicate))))
+
+;;;###autoload
+(defun +flyspell|init-predicate ()
+  "TODO"
+  (when-let* ((pred (assq major-mode +flyspell--predicate-alist)))
+    (setq-local flyspell-generic-check-word-predicate (cdr pred))))