feat: Flatpak support in the installer

[CosmoCreeper]

Would be nice to add flatpak support in the installer before the next release.

[PixsaOJ]

before was sine-flatpak.sh script as i remember. I still have it in my downloads

[CosmoCreeper]

Yes, but i think it would be a better idea to implement it into the auto-installer itself.

[Ctrlpillar]

that makes sense now i was racking my head over this
(linux-x64 btw, ubuntu)

for most ppl it auto enters right
even after manully adding the path it saying saying (Segmentation fault (core dumped) ./sine-linux-x64)
i was losing it :_)