Dark Mode support in Windows

[pento]

Electron doesn't support Dark Mode in Windows, but we can manually check the registry for the setting.

Following Firefox's implementation, the setting is stored in the SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize\AppsUseLightTheme key. It's only available on Windows 10.

There are various NPM modules for reading Windows registry values, unfortunately, none of them appear to implement watching, so we won't know when the value changes.

I suspect the best option might be to see if VSCode folks are interested in that feature being added to their module. If not, we can fork it and add it ourselves.