dev: add nix flake environment

[ElysaSrc]

I've added a flake-ified version of https://github.com/flomonster/osrd-nix-config.
You can fire up a dev shell by using nix develop. (You might need to enable experimental flags on nix to use flakes).

This is still a work in progress, here are some elements that should be discussed:

• I've put the packages files in a nix/ folder.
• I've used alejandra for linting nix files.
• It provides the scripts in the scripts/ folder as commands that can be directly called with their names (./script/generate-infra.sh will be generate-infra directly in the dropped shell).

[flomonster]

I'm not a Nix expert but it seems to work fine 👌
I just spotted the loss of autocompletion for the cargo commands.

[ElysaSrc]

I'm not a Nix expert but it seems to work fine 👌 I just spotted the loss of autocompletion for the cargo commands.

For the autocompletion on cargo commands, I'm searching but I'm not quite sure why it's doing this.

I've taken into account your suggesstions, just changed the nodePackages with a version that ensure the same nodejs version is used accross all "globally" installed packages.

[ElysaSrc]

The only changes, since the validation of @multun, is the addition of a shell.nix that leverages https://github.com/edolstra/flake-compat (edolstra is the creator of nix).
This allows extensions like https://marketplace.visualstudio.com/items?itemName=arrterian.nix-env-selector for vscode to leverages the devShell dropped by the flake.

It simply is a retrocompatibility layer.