Alias Physics Gun -> Gravity Gun

BUR #38715 has been approved by @nonamethanks.

create alias physics_gun -> gravity_gun

Physics Gun from Garry's Mod uses the model of charged Gravity Gun from Half-Life 2. It might be useful as an alias.

Worth noting there's another weapon called physgun, but it's unlikely we'll get art of it since it's cut content.

The bulk update request #38715 (forum #341889) has been approved by @nonamethanks.