feat(format): add :lang sml formatter

modules/lang/sml/config.el

@@ -4,6 +4,7 @@
   :mode "\\.s\\(?:ml\\|ig\\)\\'"
   :config
   (set-repl-handler! 'sml-mode #'run-sml)
+  (set-formatter! 'smlformat '("smlformat") :modes '(sml-mode))
 
   ;; don't auto-close apostrophes (type 'a = foo) and backticks (`Foo)
   (sp-with-modes 'sml-mode

modules/lang/sml/doctor.el

@@ -0,0 +1,5 @@
+;;; lang/sml/doctor.el -*- lexical-binding: t; -*-
+
+(when (modulep! :editor format)
+  (unless (executable-find "smlformat")
+    (warn! "Couldn't find smlformat. Formatting will be disabled.")))