The legacy M.core.event.FORM_SUBMIT_AJAX ecent has been replaced with a
new core_form/events::formSubmittedByJavascript native DOM event.

The new event can be listened to at any point in the DOM using the
following syntax:

```
import {eventTypes} from 'core_form/events';

document.addEventListener(eventTypes.formSubmittedByJavascript, handler);
```

A backward-compatabibility layer is included to ensure that any
legacy YUI event triggered on a form is still respected and the new
native event is also fired.

A similar handler is also included to ensure that any legacy YUI event
listener is still called with the same arguments.

These legacy bridges will be removed after Moodle 4.3.