Move easy-mmode-define-navigation logic to helper functions

Move easy-mmode-define-navigation logic to helper functions

The functions defined by easy-mmode-define-navigation are useful
even if the easy-mmode-define-navigation macro is not used.
Let's take a step towards exposing them by moving them out as
helpers.

This also makes the macro much easier to modify and work on.

* lisp/emacs-lisp/easy-mmode.el (easy-mmode--prev)
(easy-mmode--next): Add (bug#73172).
(easy-mmode-define-navigation): Use easy-mmode--prev and
easy-mmode--next.

(cherry picked from commit da1416fc6998718d1e36e32961b4415045949632)