Add PlayerSkinRealName as an ACS function

Details

This adds a new ACS function for map makers to use: PlayerSkinRealName(). This functions similarly to PlayerSkin() in which it returns the skin's real name rather than the internal name that it normally uses; it's good for integrating into character dialogue by allowing them to directly address the character's name.

This is meant to go along with the corresponding merge request in the Ring Racers repository. You can see it in action there.