mu4e: Place provide forms in their own sections

Use "_" as the title of that section so that it is less distracting
when sections are collapsed to get an overview of the library.

Using a separate section is useful because it reduces the risk of
accidentally into the middle of a library.

mu4e/mu4e-lists.el

@@ -97,5 +97,6 @@ be used as the shortname."
   :group 'mu4e-headers
   :type '(repeat (regexp)))
 
+;;; _
 (provide 'mu4e-lists)
 ;;; mu4e-lists.el ends here