Make "_positionCalendar" public to handle page scroll

[marc-mabe]

Hello,

The calendar does not handle page scrolling in many cases as it's rendered as an overlay on the body element using the current element position. This does not get updated of the page gets scrolled or any of the parent elements get scrolled.
In the project I'm working on I found a workaround to actually reposition the calendar on scroll event but this _positionCalendar method is not documented nor public (indicated by _ prefix).

As it's useful to manage calendar position on scroll I would suggest to make it public.

This is what I have currently:

    private updatePosition() {
        /* eslint-disable no-underscore-dangle */
        this.flatpickerStart?._positionCalendar();
        this.flatpickerEnd?._positionCalendar();
    }