checkers/spell: add doctor.el; no-op without aspell

Aspell is a hard dependency of this module, even if you use hunspell as
a backend. The spellchecker will simply bow out if aspell isn't present
on your system at startup.

modules/checkers/spell/doctor.el

@@ -0,0 +1,8 @@
+;;; checkers/spell/doctor.el -*- lexical-binding: t; -*-
+
+(unless (executable-find "aspell")
+  (warn! "Couldn't find aspell executable; spell checker will not work"))
+
+(when (featurep! +hunspell)
+  (unless (executable-find "hunspell")
+    (warn! "Couldn't find hunspell executable; spell checker will not work")))