Add `Iterator.prototype.concat` to adventure pack

It would be nice to be able to concatenate two iterators!

See [`Array.prototype.concat()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat) for inspiration from an Array counterpart.

Start by locating the Iterator implementation and existing iterator methods in the adventure pack. Read the linked Array.prototype.concat reference to clarify the intended behavior for combining two iterators. Done means Iterator.prototype.concat is available and concatenates the requested iterators consistently with the project’s existing conventions.

Written by the indexing model from the issue text.