Error: Property onLine does not have access type get

[CSchulz]

Describe the bug
If you are switching from jsdom to happy-dom you will find differences in the implementation of how the navigator is used.

With jsdom you are able to use some spy on the navigator, happy-dom does not support it:

jest.spyOn(window.navigator, 'onLine', 'get').mockReturnValue(true);

To Reproduce
Add the code line to a spec and run it.

Expected behavior
A switch between jsdom and happy-dom should be possible without any changes in the source code.

Additional context
Compare with jsdom
https://github.com/jsdom/jsdom/blob/master/lib/jsdom/living/navigator/Navigator-impl.js
https://github.com/jsdom/jsdom/blob/master/lib/jsdom/living/navigator/NavigatorOnLine-impl.js