Add support for `desktop-notifier`

:loudspeaker: What is the name of the service?

Add an adapter for desktop-notifier by @samschott.

:link: Provide some details about the service that can help with it's development.

• Homepage: https://github.com/samschott/desktop-notifier
• Documentation: https://desktop-notifier.readthedocs.io/

:bulb: Anything else?

@portalzine made a corresponding contribution at https://github.com/jpmens/mqttwarn/pull/581, replacing the macOS-specific service plugin of mqttwarn. I love it.

As a followup of adding support for desktop-notifier, maybe get rid of Apprise's native implementations for Desktop notifications on Linux, macOS, and Windows ^1, as they are intrinsically harder to maintain than an adapter for a single library which already might do the job well enough.