Add Stadia as a platform

[alkazar]

Blocked: The Arch chromium package does not launch, neither does the AppImage version of chromium.

The flatpak chromium launches, but cannot support controllers due to limitations of flatpak.

[AsciiWolf]

There's now a Chromium Flatpak on Flathub, maybe it could be used as a base.

[alkazar]

As mentioned in the comment, the chromium flatpak does not work with controllers making it useless.

I did figure out a way to get the app image version of chromium to work now though, so this can move forward.