[Tracking]: Indexer API

[JReinhold]

Indexer API Tracking issue

RFC: #23176

1. Indexer API

Beta Give feedback

1. Add tests to existing indexer functionality
2. Implement changes to the indexer API and expose types
3. Migrate existing story indexers to new indexer API
4. Remove old experimental indexer API
5. Mark new API as experimental

2. Documentation

Beta Give feedback

1. Document new indexer API (in collaboration with DX)
2. Announce new indexer API, describing existing usages and potential future applications (in collaboration with DX) Postponed until docs indexers are available too
3. Write migrations notes from storyIndexers to indexers
4. Enable deprecations (warnings, @deprecated JSDocs) for storyIndexers
5. Deprecate storiesOf
6. Update the RFC to match the "final" API and decisions

Follow-up tasks

Beta Give feedback

1. Migrate addon-svelte-csf indexer to the new API -- potentially call for contribution?: Migrate to new story indexer API addon-svelte-csf#147
2. Reach out to maintainer for storybook-vue-addon to migrate existing indexer to use the new API: Migrate to new story indexer API tobiasdiez/storybook-vue-addon#87