PlaceholderAPI support

To use following placeholders you need:

• WorldSystem 2.4.39 or higher
• PlaceholderAPI

Placeholder	Description
%worldsystem_has_world%	Returns if a player has created a world yet.
%worldsystem_is_creator%	Returns if a player is the owner of the world he is on.
%worldsystem_player_world_name%	Returns the full name of the world the player belongs. ⚠️ Returns none if he does not have a world.
%worldsystem_world_loaded%	Returns if the world of a player is loaded. ⚠️ Returns none if he does not have a world.
%worldsystem_display_world_name%	Returns world name player is on if it is not a WorldSystem world. If so, then the owner's name gets returned.