feat: make bin/doom profile aware

- Fixes Doom's former inability to (trivially) juggle multiple profiles
  based on the same EMACSDIR (see #6593).
- Adds '--profile NAME' switch to bin/doom (also recognized
  $DOOMPROFILE).
- Adds new doom-profile* variables. These will eventually replace
  doom-{local,etc,cache}-dir and doom-{autoloads,env}-file.

This is intentionally messy to ensure backwards compatibility for a
little while longer. This will be fixed over the next couple weeks.

Ref: #6593

core/cli/sync.el

@@ -49,7 +49,10 @@ OPTIONS:
   (add-hook 'kill-emacs-hook #'doom-sync--abort-warning-h)
   (when jobs
     (setq native-comp-async-jobs-number (truncate jobs)))
-  (print! (start "Synchronizing your config with Doom Emacs..."))
+  (print! (start "Synchronizing %S profile..." )
+          (if doom-profile
+              (car (split-string doom-profile "@"))
+            "default"))
   (unwind-protect
       (print-group!
        (when (and (not noenvvar?)