Make macos_ui usable on Linux and Windows

[stMerlHin]

Use case

Sometimes, we would like to use flutter to create applications in the style of MacOS applications on Linux and Windows or even to be able to contribute without necessarily having a Mac available. The problem currently is that, when we launch the demo application on Linux for example, the colors of certain widgets do not correspond to the theme of the application. I know macos_ui is not designed to support Linux and Windows but it seems to work fine on the web.

Proposal

Make macos_ui work on Windows and Linux just like it does on the web. To do this, it would be enough (I'm probably saying nonsense) to treat all other platforms as if they were the web as long as we are not on MacOS.