Rule: add story-exports

[yannbf]

Issue: #42

What Changed

Added a new rule story-exports to enforce defining at least one named export in a story file.

Checklist

Check the ones applicable to your change:

• Ran yarn update-all
• Tests are updated
• Documentation is updated

Change Type

Indicate the type of change your pull request is:

• maintenance
• documentation
• patch
• minor
• major

[yannbf]

@shilman two questions:

• is story-exports a good name or maybe named-exports is better? I chose story-exports because you can have non-story exports in story files.

• I'm not sure what to do with the output of the auto fix. I added a CSF3 example for now. A few options that we can choose:
  1 - Only report and not autofix the issue
  2 - Report and autofix with export const Default = {} and let people deal with the story (at least for now)
  3 - Report and autofix with export const Default = {} and add a comment on top with a link: https://storybook.js.org/docs/react/writing-stories/introduction
  4 - Add a setting field to ESLint so people define a framework (maybe CSF version as well), and based on the framework we can provide a better autofix and a correct docs link to the specific framework

[shilman]

• story-exports WFM
• let's do option1 for now and then do option3 as part of the CSF3 launch

[github-actions]

🚀 PR was released in v0.4.0 🚀