translate method
Implementation
@override
RectRegion translate(int x, int y) => RectRegion(
startRank: startRank != null ? startRank! + y : null,
endRank: endRank != null ? endRank! + y : null,
startFile: startFile != null ? startFile! + x : null,
endFile: endFile != null ? endFile! + x : null,
);