Add HighContrast White and Dark theme

[prateekmedia]

Adwaita theme also has HighContrast variants, It would be good for this package to also support them. It will also be useful for android with amoled screen as HighContrast Dark would be blacker.

[prateekmedia]

@MalcolmMielle Can you give me access to adwaita publication on pub.dev, my mail is prtksunal at gmail dot com

[MalcolmMielle]

Sure. We should also move the github action to this one: https://github.com/dart-lang/ecosystem/tree/main/pkgs/firehose

Then we will be able to publish directly from github