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-headers.el

@@ -1892,6 +1892,6 @@ other windows."
       (kill-buffer)
       (mu4e~main-view))))
 
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; _
 (provide 'mu4e-headers)
 ;;; mu4e-headers.el ends here