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'.

lisp/lib/config.el

@@ -9,24 +9,24 @@
 
 ;;;###autoload
 (defun doom/open-private-config ()
-  "Browse your `doom-private-dir'."
+  "Browse your `doom-user-dir'."
   (interactive)
-  (unless (file-directory-p doom-private-dir)
-    (make-directory doom-private-dir t))
-  (doom-project-browse doom-private-dir))
+  (unless (file-directory-p doom-user-dir)
+    (make-directory doom-user-dir t))
+  (doom-project-browse doom-user-dir))
 
 ;;;###autoload
 (defun doom/find-file-in-private-config ()
-  "Search for a file in `doom-private-dir'."
+  "Search for a file in `doom-user-dir'."
   (interactive)
-  (doom-project-find-file doom-private-dir))
+  (doom-project-find-file doom-user-dir))
 
 ;;;###autoload
 (defun doom/goto-private-init-file ()
   "Open your private init.el file.
 And jumps to your `doom!' block."
   (interactive)
-  (find-file (expand-file-name "init.el" doom-private-dir))
+  (find-file (expand-file-name "init.el" doom-user-dir))
   (goto-char
    (or (save-excursion
          (goto-char (point-min))
@@ -37,13 +37,13 @@ And jumps to your `doom!' block."
 (defun doom/goto-private-config-file ()
   "Open your private config.el file."
   (interactive)
-  (find-file (expand-file-name "config.el" doom-private-dir)))
+  (find-file (expand-file-name "config.el" doom-user-dir)))
 
 ;;;###autoload
 (defun doom/goto-private-packages-file ()
   "Open your private packages.el file."
   (interactive)
-  (find-file (expand-file-name "packages.el" doom-private-dir)))
+  (find-file (expand-file-name "packages.el" doom-user-dir)))
 
 
 ;;