feat(Worldborder): various features

[Amgelo563]

• Add placeholder list via #getPlaceholders()
• Add %worldborder_fromWorld_{worldName}_(argument)%, for instance %worldborder_fromWorld_{world_nether}_size%.
• Add _center, _center_x and _center_z.
• Add more descriptive errors when missing arguments or player.
• Bump version to 1.2.0.