Persist scratch buffer point, mode & contents

Also renames doom-scratch-buffer-major-mode ->
doom-scratch-initial-major-mode, since it only affects the initial
buffer now.

This was designed to be backwards compatible; you won't lose your
scratch buffers from this update. Though I may remove the old format in
3.1.

core/autoload/buffers.el

@@ -36,7 +36,7 @@ it if it doesn't exist).")
 
 
 ;;
-;; Functions
+;;; Functions
 
 ;;;###autoload
 (defun doom-buffer-frame-predicate (buf)