Add a CSV file iterator that uses the BatchLogic

[naxoc]

This adds a CSV iterator a lot like the JsonIterator we already have. It uses BatchLogic args so it should be easy to use without having to do math.

How to test

Test that all lines are read correctly passing in nothing, --start=x and/or --end=x. Use a .csv file something like this:

        $csv_file_path = $assoc_args[ $this->csv_input_file['name'] ];
        $batch_args = $this->csv_iterator->validate_and_get_batch_args_for_file( $csv_file_path, $assoc_args, ',' );
        $counter    = 0;
        foreach ( $this->csv_iterator->batched_items( $csv_file_path, ',', $batch_args['start'], $batch_args['end'] ) as $row ) {
}

To use the BatchLogic, register a command something like this:

    private array $csv_input_file = [
        'type'        => 'assoc',
        'name'        => 'csv-input-file',
        'description' => 'Path to CSV input file.',
        'optional'    => false,
    ];

    public function register_commands(): void {
        WP_CLI::add_command(
            'newspack-content-migrator my-test-thing',
            [ $this, 'cmd_test-thing' ],
            [
                'shortdesc' => 'Test the thing!.',
                'synopsis'  => [
                    $this->csv_input_file,
                    ...BatchLogic::get_batch_args(),
                ],
            ]
        );
    }

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