Ability to set different label prefixes for recurring and individual events

Problem/Motivation

Sites using this module in conjunction with smart_date_recur may want to use different text prefixing the Add to Calendar links for the output that shows event recurrence rule and the individual event occurrence list. For example, for the links that include the recurrence rule, text such as "Add all occurrences to Calendar" or "Add recurring event to Calendar" may be preferred, while text for the individual occurrences such as "Add individual instance to Calendar" may be preferred.

An ideal way to achieve this would be to make the "Add to Calendar" text currently hardcoded in addtocal-links.html.twig a configuration option in the plugin.

Since the provides separate configuration options already for the recurring event and the instance display, this should only involve adding a single configuration option for the prefix text.