* Use dark-light on Mac and Windows
dark-light has a nasty problem on Linux: https://github.com/frewsxcv/rust-dark-light/issues/17
So we made dark-light opt-in in https://github.com/emilk/egui/pull/1437
This PR makes dark-light a default dependency again,
but only use it on Max and Windows.
This is controlled with the new NativeOptions::follow_system_theme.
If this isn't enabled, then NativeOptions::default_theme is used.
* Add eframe::WebOptions