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'.

bin/doom

@@ -111,7 +111,7 @@
 
 ;; Load $DOOMDIR/init.el, to read the user's `doom!' block, and so users can
 ;; customize things early, if they like.
-(load! doom-module-init-file doom-private-dir t)
+(load! doom-module-init-file doom-user-dir t)
 
 
 ;;
@@ -299,7 +299,7 @@ SEE ALSO:
               (load! cli-file path t)))))
 
       (doom-log "Loading $DOOMDIR/cli.el")
-      (load! cli-file doom-private-dir t))
+      (load! cli-file doom-user-dir t))
 
     ;; Allow per-project Doom settings in .doom files.
     (let (doomrc)