PlaceholderAPI support

If you have PlaceholderAPI installed you can use the following placeholders:

• %anturniaquests_questscompleted% - This placeholder shows the player how many quests he already completed in total.

• %anturniaquests_quest_\<questID>% - This placeholder shows the player how often he already completed the quest with the id <questID>.

• %anturniaquests_killedentities% - This placeholder shows the player how many entities he already killed.

• %anturniaquests_killedentities_\<entityID>% - This placeholder shows the player how many entities of the type <entityID> he killed, you can find a list of valid entity id's here: https://codearray.dev/docs/minecraft-stuff#entities.