feat: add GameRules stub with PositionStatus enum

modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala

@@ -0,0 +1,17 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.*
+
+enum PositionStatus:
+  case Normal, InCheck, Mated, Drawn
+
+object GameRules:
+
+  /** True if `color`'s king is under attack on this board. */
+  def isInCheck(board: Board, color: Color): Boolean = false
+
+  /** All (from, to) moves for `color` that do not leave their own king in check. */
+  def legalMoves(board: Board, color: Color): Set[(Square, Square)] = Set.empty
+
+  /** Position status for the side whose turn it is (`color`). */
+  def gameStatus(board: Board, color: Color): PositionStatus = PositionStatus.Normal