Make session persistence module agnostic

They've been removed from feature/workspaces and moved into
core/autoload/sessions, which falls back to desktop.el if persp-mode
isn't present. This also offers a substantial speed up to
restart+restoring and restoring sessions in general.

Also fixes #1210, where the newly spawned frame after doom/restart
wasn't focused.

Introduces the following commands:

- doom/restart
- doom/restart-and-restore
- doom/quickload-session
- doom/quicksave-session
- doom/load-session
- doom/save-session
- +workspace/restore-last-session (alias for doom/quickload-session)

And removes

- +workspace/load-session
- +workspace/save-session
- +workspace/load-last-session (renamed to +workspace/restore-last-session)
- +workspace/restart-emacs-then-restore (replaced by doom/restart-and-restore)
- :ss (ex command)
- :sl (ex command)

core/cli/autoloads.el

@@ -37,9 +37,8 @@ it exists."
   (print! (bold (green "\nFinished!")))
   (message "If you have a running Emacs Session, you will need to restart it or")
   (message "reload Doom for changes to take effect:\n")
-  (when (fboundp '+workspace/restart-emacs-then-restore)
-    (message "  M-x +workspace/restart-emacs-then-restore"))
-  (message "  M-x restart-emacs")
+  (message "  M-x doom/restart-and-restore")
+  (message "  M-x doom/restart")
   (message "  M-x doom/reload"))
 
 (defun doom--do-load (&rest files)