feat: add fast element registry export path

[janechu]

Pull Request

📖 Description

Adds a focused @microsoft/fast-element/registry.js path export for FAST element definition lookups. The new entrypoint exposes fastElementRegistry, FASTElementDefinition, and the supporting registry types without requiring consumers to import from the full root entrypoint.

This also updates the registry API release tags, generated API docs, the package/website path-export docs, and FAST Element design documentation.

👩‍💻 Reviewer Notes

The registry was previously root-exported but documented as internal. This change makes the supporting TypeDefinition and TypeRegistry types public so the public registry export has a coherent API Extractor surface.

📑 Test Plan

• npm run build
• npx playwright test src/declarative/extension-subpaths.pw.spec.ts --project=chromium from packages/fast-element
• npm run checkchange -- --branch origin/releases/fast-element-v3

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.