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/editor/file-templates/autoload.el

@@ -92,8 +92,8 @@ evil is loaded and enabled)."
              (match-string 1 path))
             ((file-in-directory-p path doom-emacs-dir)
              (file-relative-name path doom-emacs-dir))
-            ((file-in-directory-p path doom-private-dir)
-             (file-relative-name path doom-private-dir))
+            ((file-in-directory-p path doom-user-dir)
+             (file-relative-name path doom-user-dir))
             ((abbreviate-file-name path))))))
 
 ;;;###autoload

modules/editor/file-templates/config.el

@@ -116,7 +116,7 @@ information.")
 
 (defun +file-templates-in-emacs-dirs-p (file)
   "Returns t if FILE is in Doom or your private directory."
-  (or (file-in-directory-p file doom-private-dir)
+  (or (file-in-directory-p file doom-user-dir)
       (file-in-directory-p file doom-emacs-dir)))
 
 (defun +file-template-p (rule)

modules/editor/snippets/config.el

@@ -1,6 +1,6 @@
 ;;; editor/snippets/config.el -*- lexical-binding: t; -*-
 
-(defvar +snippets-dir (expand-file-name "snippets/" doom-private-dir)
+(defvar +snippets-dir (expand-file-name "snippets/" doom-user-dir)
   "Directory where `yasnippet' will search for your private snippets.")