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/lang/markdown/config.el

@@ -53,6 +53,8 @@ capture, the end position, and the output buffer.")
     (add-to-list 'org-src-lang-modes '("md" . markdown)))
 
   :config
+  (set-flyspell-predicate! '(markdown-mode gfm-mode)
+    #'+markdown-flyspell-word-p)
   (set-lookup-handlers! '(markdown-mode gfm-mode)
     ;; `markdown-follow-thing-at-point' may open an external program or a
     ;; buffer. No good way to tell, so pretend it's async.