Add Navigator.onLine

[toastal]

Prerequisites

• Before opening a pull request, please check the HTML standard (https://dom.spec.whatwg.org/). If it doesn't appear in this spec, it may be present in the spec for one of the other purescript-web projects. Although MDN is a great resource, it is not a suitable reference for this project.

https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-online-dev

Description of the change

While not 100% reliable, this can be used as a part of a whole related to getting the real connectivity status of the browser.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)