Add missing implicit method from contract to interface (#814)

refactor(contracts): add missing implicit method from interface

Author: 0xjei

packages/contracts/contracts/Semaphore.sol

@@ -19,6 +19,7 @@ contract Semaphore is ISemaphore, SemaphoreGroups {
     mapping(uint256 => Group) public groups;
 
     /// @dev Counter to assign an incremental id to the groups.
+    /// This counter is used to keep track of the number of groups created.
     uint256 public groupCounter;
 
     /// @dev Initializes the Semaphore verifier used to verify the user's ZK proofs.

packages/contracts/contracts/interfaces/ISemaphore.sol

@@ -55,6 +55,10 @@ interface ISemaphore {
         uint256[8] points
     );
 
+    /// @dev Returns the current value of the group counter.
+    /// @return The current group counter value.
+    function groupCounter() external view returns (uint256);
+
     /// @dev See {SemaphoreGroups-_createGroup}.
     function createGroup() external returns (uint256);