Support for Facets

[sharunkumar]

For supporting links/mentions/urls when creating a post: https://docs.bsky.app/docs/advanced-guides/post-richtext#producing-facets

Reference implementation: https://github.com/bluesky-social/atproto/blob/011e73ec05b0db34f7c1d12382ec46e910e2d4dd/packages/api/src/rich-text/detection.ts#L13