added a debug option to go-syncuser

* added an admin dashboard settings page for go-syncuser where we can set the debug option
* slog progress from go-syncuser's cron function when the debug option is set

see https://github.com/GigaOM/legacy-pro/issues/4020
`wp_die()`, which will eventually call `_ajax_wp_die_handler()`

### OR

`wp_send_json()` or its friends, `wp_send_json_error()` and `wp_send_json_success()`