Svelte: Add experimental portable stories support

[yannbf]

Closes #25878

What I did

This PR introduces the portable stories API to Svelte. It works slightly different than in other renderers, because of how Svelte testing libraries expect their components. Mostly because the portable story provides Component and props properties to be used in the test.

// Button.test.ts
import { test } from 'vitest';
import { render } from '@testing-library/svelte';
import { composeStory } from '@storybook/svelte';

import meta, { Primary } from './Button.stories';

test('renders and executes the play function', async () => {
  const PrimaryStory = composeStory(Primary, meta);

  // First, render the story
  render(PrimaryStory.Component, PrimaryStory.props);

  // Then, execute the play function
  await PrimaryStory.play();
});

There are a few limitations though. Users can't pass custom props to their components in the test like they normally would with Svelte components directly:

// ❌ This is not possible
render(PrimaryStory.Component, { ...Primary.props, foo: 'bar' });

They can only get the component to render with props that come from args. If they really want to provide custom props, they should do it while composing the story:

import { composeStory } from '@storybook/svelte';
import meta, { Primary } from './Button.stories';

// ✅ This is possible
const PrimaryStory = composeStory({ ...Primary, args: {...Primary.args, foo: 'bar' } }, meta);

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

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

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

• Make sure this PR contains one of the labels below:

  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.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

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>

[github-actions]

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🚫	PR title must be in the format of "Area: Summary", With both Area and Summary starting with a capital letter Good examples: - "Docs: Describe Canvas Doc Block" - "Svelte: Support Svelte v4" Bad examples: - "add new api docs" - "fix: Svelte 4 support" - "Vue: improve docs"

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[kylegach]

Docs look good!

I pushed a commit to better call out the unique way Svelte needs to override props for an individual story.