Addon-docs: Support non-story exports in module story format

Storybook docs is shipping with a new story format for ES6 modules based on this gist that treats all named exports as stories.

Opening this issue as a place to discuss how we might also support non-story exports. This came up in https://github.com/storybookjs/storybook/issues/6995 but it is a generally useful feature for exporting reusable story data.

Here’s a simple example for the discussion:

export default {
  title: 'components/Button'
}; // stories metadata
export normal = () => <Button>Hello button</Button> // a story
export someData = { foo: 1, bar: 2 } //????

Three proposals so far from @tmeasday / @ndelangen / @shilman include marking individual stories, specifying story exports, and filtering non-story exports.

Marking stories

export normal = () => <Button>Hello button</Button>;
normal.isStory = true;

Which could be stylized as:

import { story } from '@storybook/story';
export normal = story(() => <Button>Hello button</Button>);

Specifying story exports

export default {
  title: 'components/Button'
  storyExports: ['normal'] // defaults to all exports as stories
};
...

Filtering non-story exports

export default {
  title: 'components/Button'
  ignoreExports: ['someData'] // defaults to all exports as stories
};
...

[tmeasday]

I think if we can make the black/whitelist easily configurable via params, then we shouldn’t have a default – to me that falls under the purview of magic that the user might not know about and find hard to debug (why is my story _iLoveUnderscores not showing up?)

[bdombro]

Hint: To use the new feature you can now allowList using includeStories: ['myStory1', ...] or blockList using excludeStores: ['myNonStoryExport', ...] in export default