emacs: Add hooks for REPL operations.

* emacs/guix-backend.el (guix-before-repl-operation-hook,
  guix-after-repl-operation-hook, guix-repl-operation-p): New variables.
  (guix-repl-operation-success-message, guix-repl-output-filter): New
  procedures.
  (guix-start-repl): Adjust for using 'guix-repl-output-filter'.
  (guix-eval-in-repl): Honor 'guix-before-repl-operation-hook'.