[custom-field] Checking the required by pressing tab does not work

[shadikhani]

The components in the custom-field cannot be validated by pressing the tab.

[web-padawan]

Marked as enhancement because IIRC this is how the component was designed.

We added the internal-tab event later on to implement validation in vaadin-date-time-picker.
So we probably could have similar logic in vaadin-custom-field.