Enable eslint-plugin-storybook

Storybook now provides an official ESLint plugin to ensure stories are written properly: https://github.com/storybookjs/eslint-plugin-storybook.

It currently reports errors for all stories that haven't been migrated to CSF yet, which is expected.

Let's enable this plugin sometime soon. This could be done now by ignoring offenses in non-CSF stories, or we could also wait until the migration is done.

Recommended setup in .eslintrc.yml:

extends:
  - 'plugin:storybook/csf'