API: Add API access to sidebar renderLabel

[shilman]

Closes N/A

What I did

Added a way to access parent/child node story metadata from the renderLabel function.

Here's an example of how the new API can be used to propagate story tags up to their wrapping components (the use case I had in mind for this feature):

// manager.ts
import { addons } from '@storybook/manager-api';
import type { API, LeafEntry, HashEntry } from '@storybook/manager-api';

const SYSTEM_TAGS = ['dev', 'autodocs', 'test'];

const findComponentTags = (stories: LeafEntry[]) => {
  const allTags = stories.flatMap((story) => story.tags);
  const tagToCount = allTags.reduce(
    (acc, tag) => {
      acc[tag] = (acc[tag] || 0) + 1;
      return acc;
    },
    {} as Record<string, number>
  );
  return Object.entries(tagToCount)
    .filter(([tag, count]) => count === stories.length && !SYSTEM_TAGS.includes(tag))
    .map(([tag]) => tag);
};

addons.setConfig({
  sidebar: {
    renderLabel: (item: HashEntry, api: API) => {
      if (item.type !== 'component') return item.name;
      const tags = findComponentTags(item.children.map((childId) => api.getData(childId)));
      const tagsLabel = tags.length > 0 ? ` [${tags.join(', ')}]` : '';
      return `${item.name}${tagsLabel}`;
    },
  },
});

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

Documentation

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

🦋 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>

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[tmeasday]

LGTM