[Feature] Flatpak support

[AkazaRenn]

https://flathub.org/apps/org.ryujinx.Ryujinx this is where the Flatpak version of Ryujinx was originally published. I'm wondering if it can be taken over for continuing the release.

[regginator]

Possibly in the future, I'd be open to requesting a Flathub scope for dev.ryujinx.Ryujinx once we're using proper semver again, though for now we can consider adding a .flatpakref to releases. We've also added AppImage build infrastructure, too 😄

[nanopone]

i think its worth pointing out that flatpaks can be distributed as single-file bundles similar to appimages and don't actually need to be published to a repo like flathub, making it basically a no-brainer considering appimage's problems