feat: ion-datetime with date range

# Feature Request
It would be great if the new ion-datetime would support a range feature (like for example in a hotel booking webpage) where you can provide a start and enddate and it shows on the calendar component.


**Ionic version:**

[ ] **4.x**
[ ] **5.x**
[x] **6.x**

Start by reading the Ionic documentation for ion-datetime and inspecting the existing date-picker behavior. Define how a start and end date should be selected and displayed on the calendar, then verify that the component supports the requested hotel-booking-style range interaction.

索引モデルが issue の本文から書いたものです。