Feat/prevent vanished trading

[bennycallanan]

Prevents trading with vanished players by treating them as offline.

• Added vanish detection using player.canSee(player)
• Prevents trade requests when either player is vanished
• Uses same error message as offline players: "The player [name] can not be found!"
• Configurable via prevent-vanished-trading option (default: false)
• Compatible with most vanish plugins