chore: add nix flake

[joscha]

This adds a nix flake similar to the one we have in https://github.com/OpenAPITools/openapi-generator

It installs yarn 1.x and node 18 w/o npm and adds a direnv directive.

update: we need npm, it seems we use it here:

openapi-generator-cli/package.json

Line 54 in 113ee84

"publish": "npm run build && npm publish ./dist/apps/generator-cli",

in addition to yarn

The current lock file sets this to:

node --version
v18.20.4
yarn --version
1.22.22
npm --version
10.7.0

same as in the current master: https://github.com/OpenAPITools/openapi-generator-cli/actions/runs/11091950012/job/30816360053

[github-actions]

🎉 This PR is included in version 2.14.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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