Add support for navigation (back, forward)

The browser history is already maintained in the ProgramTest model (inspectable with `expectBrowserUrl` and `expectBrowserHistory`). This issue would add `back` and `forward` to `SimulatedEffect.Navigation` to parallel the functions in https://package.elm-lang.org/packages/elm/browser/latest/Browser-Navigation . Simulating those events should have the effect of calling `routeChangeHelper` with the new URL if the action can be performed, or failing the test if not.

Implementing `forward` will require adding the forward "history" to the ProgramTest model.

If you are looking to contribute, feel free to make a PR that only implements one of these functions. This is marked "good first issue", but you'd need to have a good understanding of using elm-program-test and of https://elm-program-test.netlify.com/cmds.html or else this probably won't be easy.

Start by inspecting the ProgramTest model, its existing browser-history behavior through expectBrowserUrl and expectBrowserHistory, and SimulatedEffect.Navigation. Trace routeChangeHelper and the existing command tests or entry points, then implement either back or forward. Done means a performable action calls routeChangeHelper with the new URL, while an unavailable action fails the test; forward also requires the model's forward history.

Written by the indexing model from the issue text.