Issue #3318590 by muriqui, owenbush: Add iCalendar support

...chema/recurring_events_ical.event_ical_mapping.schema.yml

...fig/schema/recurring_events_ical.ical_property.schema.yml

modules/recurring_events_ical/recurring_events_ical.info.yml

...urring_events_ical/recurring_events_ical.links.action.yml

...ecurring_events_ical/recurring_events_ical.links.menu.yml

modules/recurring_events_ical/recurring_events_ical.module

...s/recurring_events_ical/recurring_events_ical.routing.yml

.../recurring_events_ical/recurring_events_ical.services.yml

...ring_events_ical/src/Controller/EventExportController.php

...les/recurring_events_ical/src/Entity/EventICalMapping.php

modules/recurring_events_ical/src/EventICal.php

modules/recurring_events_ical/src/EventICalInterface.php

...s/recurring_events_ical/src/EventICalMappingInterface.php

...recurring_events_ical/src/EventICalMappingListBuilder.php

...recurring_events_ical/src/Field/EventICalLinkItemList.php

...rring_events_ical/src/Form/EventICalMappingDeleteForm.php

...s/recurring_events_ical/src/Form/EventICalMappingForm.php

...rc/Plugin/Field/FieldFormatter/EventICalLinkFormatter.php

...nts_ical/src/Plugin/Field/FieldType/EventICalLinkItem.php