Multi-Date Selection Mode for Pro Forms Date Field

Please add support for selecting multiple individual dates in the Pro Forms Date field element.

Why this is useful? Many real world use cases require non-consecutive date selections, booking recurring appointments, selecting available days for a service, scheduling multi-day events with gaps, etc.

The Date field currently uses Flatpickr as its underlying date picker library. Flatpickr natively supports a mode: “multiple” option, which allows users to pick several separate dates in a single calendar interaction, without requiring a range or consecutive days.

Flatpickr already supports this mode natively. It would primarily require surfacing the option in the field settings UI and handling the serialized output value.