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/lang/emacs-lisp/config.el

@@ -8,7 +8,7 @@
 This marks a foldable marker for `outline-minor-mode' in elisp buffers.")
 
 (defvar +emacs-lisp-disable-flycheck-in-dirs
-  (list doom-emacs-dir doom-private-dir)
+  (list doom-emacs-dir doom-user-dir)
   "List of directories to disable `emacs-lisp-checkdoc' in.
 
 This checker tends to produce a lot of false positives in your .emacs.d and