Added Media.PlayMusic to the Lua API

and make use of it on the shellmaps to fix https://github.com/OpenRA/OpenRA/issues/6071.
That track is the score screen theme "Great Shot!" https://www.youtube.com/watch?v=wyxnxRbDkPw while the mission is playing "Act on Instinct" https://www.youtube.com/watch?v=n4AUY-v1nsE