Replace option align:number with a callback

[davidjerleke]

Feature request is related to

• embla-carousel (core package)

Describe the solution you'd like (discussed in #579)

^{Originally posted by davidjerleke September 25, 2023}
@tobimori we could replace the number option with a callback function like so:

const options = {
  align: (viewSize, snapSize, index) => {
    // create your own alignment
    return yourCustomAlignment
  }
}

The benefit of this would be that you would have direct access to the size of the container (viewSize) and the current slide/slideGroup (itemSize) which is convenient because you don't have to read element dimensions yourself. Note that the function would run for each and every scroll snap. This would be even more flexible than the previous implementation and wouldn't affect bundle size.