Add Windows Support

[xmine64]

Support for notification badge added to Windows since build 10.0.10240 (First stable build of Windows 10 supports it). You can use BadgeUpdateManager API to use it. You'll need to use WinRT API but I'm not sure if it works in unpackaged apps, but since Flutter supports building MSIX packages it's possible to at least use it on packaged Flutter apps. I may implement it in my free times but I can't find any useful document on Windows plugins for Flutter.

Btw MSIX support added to Windows since build 10.0.16229 (version 1709) and WinRT support in unpackaged apps added to Windows since build 10.0.18362 (version 1903) so you won't be able to use these on Flutter on any Windows version lower than these at all.

Windows Notification API badge references:

• Badge notifications for Windows apps
• BadgeUpdateManager Class
• BadgeUpdater Class

Other Useful references:

• Windows Runtime APIs not supported in desktop apps
• Call Windows Runtime APIs in desktop apps
• Enhancing Non-packaged Desktop Apps using Windows Runtime Components