Missing invalid case in validateFen

[YusungAN]

It seems that validateFen() currently treats some invalid FEN strings as valid.
Specifically, when the side to move is already checking opposite, the function still considers the FEN valid.

For example, the following FEN should be considered invalid, but is currently treated as valid:
1n6/8/2Q5/1rK5/8/8/8/5k2 b - - 0 1

import { validateFen } from "chess.js";

const fen = "1n6/8/2Q5/1rK5/8/8/8/5k2 b - - 0 1";

const isValid = validateFen(fen);