support new "shiny-mirrors" app

[lectrode]

Per this manjaro gitlab issue, manjaro may be moving away from pacman-mirrors to the brand new shiny-mirrors

I'll probably wait a bit before implementing this depending on how quickly manjaro adopts it, just to let it mature a bit.

Once I do implement support for it, I'll need to make sure config specific to refreshing mirrors is generic enough to apply to both the old and the new mirrors solution.

Support for pacman-mirrors will likely not go away in the foreseeable future. manjaro devs would need to drop it from the repos before I even consider dropping that support.