feat: add CommandDefinition.GetTag helper

[DevD4v3]

The documentation includes a sample example:

public class MyPermissionChecker : IPermissionChecker
{
    public bool HasPermission(Player player, CommandDefinition command)
    {
        var permission = command.GetTag("permission");
        if (permission == null)
            return true; // No permission requirement
        
        return player.HasPermission(permission);
    }
}

But the GetTag method doesn't exist in CommandDefinition. This PR adds that helper.