Docblock updates
Switch from `\WP_CLI\Utils\format_items()` to the newer `\WP_CLI\Formatter->display_items()` method.
Output an empty table instead of a message when there are no cron events on the site
Remove some test code
Correct indentation
Rename `_list()` methods to `list_()`
Make `$time_format` a property on the `Cron_Event_Command` class
Introduce `wp cron event schedule` for scheduling new events
Remove `next_run` from the default list of event fields for now