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'.

.doomrc

@@ -42,7 +42,7 @@
                              (format "%s" (substring scope 1))
                            (format "*/%s" scope)))
             (list (doom-dir (dir!) "../modules/")
-                  (doom-dir doom-private-dir "modules/"))))
+                  (doom-dir doom-user-dir "modules/"))))
 
 (defun ci-check-docs-scope (scope _)
   "Allow any filename in docs/* as a scope for docs commits."