Portable stories: Add support for loaders

[yannbf]

Closes #22633

What I did

This PR introduces a new method to a composed story called load. It's an async function that will execute the storie's loaders and make sure that once it renders, the context.loaded property will be correctly set.

  it('loaders example', async () => {
    await LoaderStory.load(); // this will make the loaders be applied to the story's render function
    render(<LoaderStory />);
  });

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

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Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

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  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
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  • feature request: Introducing a new feature.
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🦋 Canary release

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core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[JReinhold]

LGTM apart from a few nits.