Persist scratch buffer across sessions

- Adds doom/open-project-scratch-buffer (persistent project scratch
  buffers)
- Prefix arg = open scratch buffer in current window, for both
  doom/open-scratch-buffer and doom/open-project-scratch-buffer.
- Rename doom/delete-scratch-files ->
  doom/delete-persistent-scratch-file
- Remove doom-scratch-buffer-display-fn
- Rename doom-scratch-files-dir -> doom-scratch-dir
- Add SPC p s keybind to open project scratch buffer

modules/ui/popup/config.el

@@ -136,7 +136,7 @@ prevent the popup(s) from messing up the UI (or vice versa)."
        :slot -1 :vslot -2 :ttl 0)
       ("^\\*Compil\\(?:ation\\|e-Log\\)"
        :vslot -2 :size 0.3 :ttl nil :quit t)
-      ("^\\*\\(?:\\(?:doom:\\)?scratch\\|Messages\\)"
+      ("^\\*\\(?:scratch\\|Messages\\)"
        :autosave t :ttl nil)
       ("^\\*Man "
        :size 0.45 :vslot -3 :ttl 0 :quit t :select t)