Migrate eslint configuration

[OlafConijn]

it seems that this repository uses eslint configuration that is not compatible with newer versions, sometimes there also is a PR stuck called 'chore(deps): update linters (major)'.

what needs to happen:
- anywhere we have an .eslintrc.json , .eslintrc.js, or embedded config in package.json, it needs to be moved to a new eslint.config.js file. you can migrate the first type using this command: npx @eslint/migrate-config .eslintrc.json.
- the --ext flag in package.json isn't supported anymore. so eslint . --ext .ts needs to become eslint .

more information: https://eslint.org/docs/latest/use/configure/migration-guide