Add `l2BlockNumber` to `IFaultDisputeGame`

packages/contracts-bedrock/contracts/dispute/FaultDisputeGame.sol

@@ -252,6 +252,13 @@ contract FaultDisputeGame is IFaultDisputeGame, Clone {
         emit Move(_challengeIndex, _pivot, msg.sender);
     }
 
+    /**
+     * @inheritdoc IFaultDisputeGame
+     */
+    function l2BlockNumber() public pure returns (uint256 l2BlockNumber_) {
+        l2BlockNumber_ = _getArgUint256(0x20);
+    }
+
     ////////////////////////////////////////////////////////////////
     //                    `IDisputeGame` impl                     //
     ////////////////////////////////////////////////////////////////

packages/contracts-bedrock/contracts/dispute/interfaces/IFaultDisputeGame.sol

@@ -64,4 +64,11 @@ interface IFaultDisputeGame is IDisputeGame {
         bytes calldata _stateData,
         bytes calldata _proof
     ) external;
+
+    /**
+     * @notice The l2BlockNumber that the `rootClaim` commits to. The trace being bisected within
+     *         the game is from `l2BlockNumber - 1` -> `l2BlockNumber`.
+     * @return l2BlockNumber_ The l2BlockNumber that the `rootClaim` commits to.
+     */
+    function l2BlockNumber() external view returns (uint256 l2BlockNumber_);
 }