Allow custom fields as title

I am taking it up.

This will work only after the related issue is fixed.

From a functional perspective this works on 10.3.10 when applied with the related fix in https://www.drupal.org/project/fullcalendar/issues/3241489 🐛 Fix fullcalendar_field_is_date() re: non-entity Views fields (e.g. 'Custom text') Needs review and there are no phpstan errors introduced. A custom field consisting of box text and a token of another field in the view was used and rendered properly.

Can't vouch if there's a tighter way to do it.

Airplane WiFi sucks but composer was finally able to patch the module. :p