Add `Restrictive.element`, managing Url changes through ports

[upsiflu]

The interface will be exactly the same as application, but:

• You need two ports, one for writing and one for receiving the Url
• You are able to mix the elm app with different islands on your page, for example if you are using Astro

Implementation: Very straightforward in Restrictive above application.

Outlook: We could actually make a builder where you construct the Restrictive app, up from a sandbox, adding features.

Discussion: https://github.com/elm/browser/blob/1.0.2/notes/navigation-in-elements.md

[upsiflu]

Prior art: https://github.com/choonkeat/elm-element-navigation