chore: Split off dev inputs into ./devshell/flake.nix

[blaggacao]

I think it's a respectful pattern to try to minimize impact on downstream flake.lock-files.

I can self-assign to this.

[kamadorueda]

Sounds cool

[kamadorueda]

I tried doing this, but then I would need to coordinate the nixpkgs versions, and find a way to re-use logic between flakes, just too much

So instead I removed a few unnecesary inputs and follows, we are now as light as possible