Added a new config value which is 'world-border-start-amount' and will allow you to configure how many blocks the border will shrink by during the first shrink. Which is determined by the 'world-border-start-shrinking-time' value.
Added a placeholder which is '<amount>' and will be replaced by the amount of blocks that the border shrinks by when used in the 'world-border-start-shrinking' and 'world-border-shrink' messages.
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