make Config.coordinatesOnSquares optional

src/config.ts

@@ -12,7 +12,7 @@ export interface Config {
   lastMove?: cg.Key[]; // squares part of the last move ["c3", "c4"]
   selected?: cg.Key; // square currently selected "a1"
   coordinates?: boolean; // include coords attributes
-  coordinatesOnSquares: boolean; // include coords attributes on every square
+  coordinatesOnSquares?: boolean; // include coords attributes on every square
   autoCastle?: boolean; // immediately complete the castle by moving the rook after king move
   viewOnly?: boolean; // don't bind events: the user will never be able to move pieces around
   disableContextMenu?: boolean; // because who needs a context menu on a chessboard