refactor: deprecate doom-private-dir for doom-user-dir

- Deprecates the doom-private-dir variable in favor of doom-user-dir.
- Renames the pseudo category for the user's module: :private -> :user.
- Renames the doom-private-error error type to doom-user-error.

Emacs uses the term "user" to refer to the "things" in user space (e.g.
user-init-file, user-emacs-directory, user-mail-address, xdg-user-dirs,
package-user-dir, etc), and I'd like to be consistent with that. It also
has the nice side-effect of being slightly shorter. I also hope
'doom-user-error' will be less obtuse to beginners than
'doom-private-error'.

modules/email/wanderlust/config.el

@@ -17,8 +17,8 @@
 
   (setq wl-demo nil
         wl-stay-folder-window t
-        wl-init-file (expand-file-name "wl.el" doom-private-dir)
-        wl-folders-file (expand-file-name "folders.wl" doom-private-dir))
+        wl-init-file (expand-file-name "wl.el" doom-user-dir)
+        wl-folders-file (expand-file-name "folders.wl" doom-user-dir))
 
   (setq wl-message-truncate-lines t
         wl-summary-width 120