Add filter for displaying total slots left

[Valentina R]

This filter woocommerce_appointments_time_slot_html can be used to change of the display for the slots left. But the client wants to always display slots left, even when all of them are still available.

Can we add a filter for it?

Requested here: https://bookingwp.com/forums/topic/how-to-show-always-the-number-of-appointments-available-on-calendar/

[Željan Topić]

Added with 4.10.5