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-compose.el

@@ -944,5 +944,6 @@ is supplied, or Transient Mark mode is enabled and the mark is active."
 (define-key mu4e-compose-mode-map
   (vector 'remap 'end-of-buffer) 'mu4e-compose-goto-bottom)
 
+;;; _
 (provide 'mu4e-compose)
 ;;; mu4e-compose.el ends here