Factory refurbished PlayMusicOnMapLoad

To fix https://github.com/OpenRA/OpenRA/issues/6071 and bring it in line with the legacy https://github.com/OpenRA/OpenRA/blob/bleed/mods/common/lua/media.lua#L11 which this aims to supersede.
This will probably need an upgrade rule (although I'm not sure if any maps/mods actually define and use music)