`emblaApi.slidesInView()` should use IntersectionObserver

[davidjerleke]

Feature request is related to

• [x] embla-carousel (core package)

Is your feature request related to an issue?

• When emblaApi.slidesInView() is called intensely on scroll, there's a performance hit.

Describe the solution you'd like

• The current implementation for slidesInView was build for the IE11 era. I'd like to get rid of the performance hit and refactor the slidesInView code so it's using IntersectionObserver instead, now that IE11 is dead.

⚠️ The slidesInView option will be replaced with the IntersectionObserver threshold so it will accept both a single number or an array of numbers:

Setup

const options = {
  slidesInView: IntersectionObserverInit['threshold'] // Which is --> number | number[] 
}

Usage

emblaApi.on('slidesInView', (emblaApi) => {
  console.log(emblaApi.slidesInView())
})

[davidjerleke]

Released with v8.0.0-rc12.