Add Nix flake

[WillPower3309]

I noticed from the readme that it appears vkdt is supportive of the nix project. Is there interest in including a nix flake in this project? This would involve a flake.nix and flake.lock file living in the project that would define and lock dependencies of the project so that nix users could build (and develop) the current main git branch, as opposed to having to use the most recent nixpkgs release. This would also have some CI advantages, if the project ever adds some more GitHub actions, which I'd also be happy to lend a hand with if there's interest.

I'd be more than happy to contribute and maintain the flake, but wanted to do an interest check before I cut a PR.

[hanatos]

hi! i'm certainly interested in better access for nix (and arch) people. this appimage business breaks more often than i'd like, and yeah.. the release is super old now and lacks all the interesting features.