Make an official Flatpak on Flathub

if @hampus-fluxer could make the flatpak himself that would be the best way forward. i'm doing my best, and have a fully functional flatpak by pulling the tarball from the official website, but as flathub requires building from source wherever possible i feel like there is little i can do without refactoring a lot of stuff to use npm instead of pnpm, which i'm in no position to do as i am really unfamiliar with the javascript ecosystem (and hampus probably chose pnpm for a reason). he knows the codebase better than anyone else so he can adapt it for flatpak-builder's lack of any real pnpm support. flathub also requires some files (.desktop file, appstream metadata, icon...) to be upstream. i highly endorse the creation of a flatpak (specifically an official one), as appimage has several security and usability pitfalls and some people i know outright refuse to use it. if anyone else wants to take a shot at making an flatpak i recommend taking a look at the fluxer-git pkgbuild on the aur for reference. if you come up with something that works and can be automated on flathub's ci, make a pr against my new-pr flathub branch to avoid making a duplicate pr on flathub's repo.