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/completion/company/config.el

@@ -180,7 +180,7 @@
 (use-package! company-dict
   :defer t
   :config
-  (setq company-dict-dir (expand-file-name "dicts" doom-private-dir))
+  (setq company-dict-dir (expand-file-name "dicts" doom-user-dir))
   (add-hook! 'doom-project-hook
     (defun +company-enable-project-dicts-h (mode &rest _)
       "Enable per-project dictionaries."