Upgrade eslint config with flat config and es module

[imagoiq]

Our current way to handle config in eslint is deprecated and should be upgraded: https://eslint.org/blog/2023/10/flat-config-rollout-plans/

Tasks

Beta Give feedback

1. Replace config with new eslint.config.js and convert to es module
2. Replace ___dirname from node to es module alternative

[imagoiq]

Following Guide for migration, I initialized the work with the branch: https://github.com/swisspost/design-system/tree/chore/2167-eslint-flat-config

But most of the eslint packages, that we use, do not support the new config yet:

• ESLint Flat Config Support typescript-eslint/typescript-eslint#7694
• feat: support @typescript-eslint/eslint-plugin ^7.0.0 stencil-community/stencil-eslint#104
• Support eslint new configuration style "flat file" angular-eslint/angular-eslint#1280 (comment)
• Support for the new FlatConfig format storybookjs/eslint-plugin-storybook#135

See the Tracking

[oliverschuerch]

We'll need to update to eslint v9, but not every dependency is ready to do this now.