Add Iterable.intersperse extension method

[ebraminio]

I needed something that could add an element between each two elements in a list as I just hardcoded that functionality in my flutter app but now have decided to revise that code so did some research about it as I didn't even know what name that has and eventually reached to https://github.com/apple/swift-algorithms/blob/main/Guides/Intersperse.md which itself is linked to https://hackage.haskell.org/package/base-4.14.0.0/docs/Data-List.html#v:intersperse and https://docs.rs/itertools/0.9.0/itertools/trait.Itertools.html#method.intersperse which felt that makes it enough popular to be added to collection. It would be ok also if you would like to rename this to something better.

Or maybe we should have two of them, intersperse that mutates a list and interspersed, the one who returns a new iterator, or maybe we should provide only one and should be named interspersed like swift or we should follow rust and Haskell instead.