Bring our eslint config and move it to own file

[LukasHirt]

Currently we are using eslint rules which were already defined by the author of the design system we are using instead of our set of rules. Let's bring it here so we are aligned with other projects and move it from package.json to own file.

[pascalwengerter]

~@LukasHirt just checked and they're quite similar now, what's missing to make them match 100%? :)~

Would extend this also to stylelint config

Guides:

• https://eslint.org/docs/developer-guide/shareable-configs

Inspiration:

• https://github.com/airbnb/javascript
• https://github.com/sourcegraph/stylelint-config

Need somebody (@kulmann ?) to create a new repo (à la, e.g., github.com/owncloud/web-lint-configs)