Added Media.PlayMusic to the Lua API

and make use of it on the shellmaps to fix https://github.com/OpenRA/OpenRA/issues/6071.
Ok. Definitely doesn't work then. I tried swapping "aoi" and "win1", and got no sound during the mission, but Act of Instinct on the score screen, so the logic is working just fine. It just doesn't play those songs for some reason. FWIW, the music player doesn't list a song by the name of "Great shot", either. Maybe my download is just borked? (although I did remove the scores.mix file and re-downloaded it).