Add notifications support to 'mpc' (Bug#73538)

Add notifications support to 'mpc' (Bug#73538)

* lisp/mpc.el (mpc-notifications, mpc-notifications-title)
(mpc-notifications-body): New option.
(mpc--notifications-id): New variable.
(mpc-notifications-notify, mpc-cover-image-find)
(mpc-cover-image-p, mpc--notifications-format): New function.
(mpc-format): Use 'mpc-cover-find' and expand docstring to
include details about the FORMAT-SPEC.
(mpc-status-callbacks): Add file callback for
notifications.

(cherry picked from commit 49084bad7990a614bdd3ea7a24ebab0fc89627e3)