Provide a version of interspersed that accepts a closure.

When doing macOS or iOS programming, a common use-case I have for `interspersed` is to insert a separator or divider view in-between a bunch of other views.

Example, given an array of views, I'd like to insert a separator view in-between each one:

```
let views = [view1, view2, view3]

let allViews = views.interspersed(using: { SeparatorView() })

```
Expected:

`[view1, separator1, view2, separator2, view3]
`

Because these are NSView, the inserted element needs to be a new instance each time, which the existing implementation does not appear to offer.

Start by locating the existing interspersed implementation and its tests in the swift-algorithms repository. Add coverage for a closure-based form that creates a distinct separator between adjacent views, and confirm the expected sequence for three input elements.

Written by the indexing model from the issue text.