[Emacs] Add s.el library

This commit adds the s.el string library for Emacs Lisp to the list of
installed packages

org/config/spacemacs.org

@@ -106,6 +106,7 @@
    | org-tree-slide      | presentation tool for org-mode                       |
    | outorg              | edit comments as Org-mode buffers                    |
    | pinentry            | enter a GPG password from Emacs                      |
+   | s                   | The long lost Emacs string manipulation library.     |
    | visual-fill-column  | allow the use of ~fill-column~ in ~visual-line-mode~ |
    | wrap-region         | easily wrap region with delimiters                   |
    | wttrin              | weather in Emacs                                     |