feat(emacs-lisp): add +emacs-lisp/change-working-buffer command

This changes what buffer `+emacs-lisp-eval` evaluates elisp in (useful
for the :tool eval module's commands, like `+eval/buffer` and
`+eval/region`, or the Evil operators on gr/gR).

Creates a scratch buffer/org src block alternative to ielm.

modules/lang/emacs-lisp/config.el

@@ -137,6 +137,7 @@ See `+emacs-lisp-non-package-mode' for details.")
 
   (map! :localleader
         :map (emacs-lisp-mode-map lisp-interaction-mode-map)
+        :desc "Set working buffer" "b" #'+emacs-lisp/change-working-buffer
         :desc "Expand macro" "m" #'macrostep-expand
         (:prefix ("d" . "debug")
           "f" #'+emacs-lisp/edebug-instrument-defun-on