refactor: deprecate doom-etc-dir for doom-data-dir

doom-etc-dir will be renamed to doom-data-dir, to better reflect its
purpose, and align it with XDG_DATA_HOME (where it will be moved to in
v3, where Doom will begin to obey XDG directory conventions more
closely).

modules/app/emms/config.el

@@ -3,7 +3,7 @@
 (use-package! emms
   :defer t
   :init
-  (setq emms-directory (concat doom-etc-dir "emms")
+  (setq emms-directory (concat doom-data-dir "emms")
         emms-cache-file (concat doom-cache-dir "emms"))
   :config
   (emms-all)

modules/app/twitter/config.el

@@ -4,7 +4,7 @@
   :commands twit
   :config
   (setq twittering-private-info-file
-        (expand-file-name "twittering-mode.gpg" doom-etc-dir)
+        (expand-file-name "twittering-mode.gpg" doom-data-dir)
         twittering-use-master-password t
         twittering-request-confirmation-on-posting t
         ;; twittering-icon-mode t