Dynamically add list filter when none exist in the original listConfig

When a controller uses the ListController behavior and no filter has been defined in its config_list.yaml, this trick will allow you to dynamically create one.

Controllers\MyController::extend(function ($controller) {
    if (!$controller->listConfig) {
        // controller uses the default value
        $controller->addDynamicProperty('listConfig', 'config_list.yaml');
    }
    $listConfig = $controller->makeConfig($controller->listConfig);
    if (!isset($listConfig->filter)) {
        // no filter defined in config_list.yaml, let's create one
        $listConfig->filter = [
            'scopes' => [
                'myScope' => [
                    'type' => 'checkbox',
                    'label' => 'Apply My Scope', 
                    'default' => false,
                    'conditions' => 'mod(id,2)',
                ],
            ],
        ];
    }
    $controller->listConfig = $listConfig;
});

Note: if a filter already exists, use the extendListFilterScopes() method. ref. https://wintercms.com/docs/backend/lists#extend-filter-scopes