Add PlaceholderAPI placeholders support

[MotikaCraft-TheJoshua]

like %stacker_isriding% = if player is riding on top of someone, it returns "true", otherwise "false"
also %stacker_isridden% = if player is being ridden by someone, it returns "true", otherwise "false"

[Zoey2936]

@DavidCraftDev

[MotikaCraft-TheJoshua]

...

[DavidCraftDev]

Hi @TheJoshue, I'm no longer actively working on Stacker at the moment, and that probably won't change for a while. I don't want to say that I will never implement it, but not in the near future, I'm sorry