Add support for stripping optional peer dependencies

[rtkt]

Hi.
Peer dependencies can be marked as optional in NPM's package.json since v7 (proof) but looks like node2nix doesn't support stripping them.

Maybe I'll patch node2nix myself and open a pull request by the end of this week but I don't guarantee it.

[lilyinstarlight]

Yeah I neglected to implement that when updating node2nix last for npm v7. Node2nix is fairly unmaintained these days, but if you need it, feel free to open a PR to this repo and then open a PR to nixpkgs to pull in the patch (and ping me to review on nixpkgs)