Add app.bsky.richtext.facet

[pfrazee]

• Add app.bsky.richtext.facet
  • Has an index which is #textSlice
  • and value which is an open union of #link and #mention
  • #link has a uri field
  • #mention has a did field
• Update app.bsky.feed.post...
  • Mark entities as deprecated but leave it in
  • Add facets as an optional field
  • This will make migration smooth — we keep supporting entities but start writing and using facets
• Update notification indexing to read the facets field but keep around the old entities field with a plan to deprecate
• Reasoning
  • Entities was created before Lexicon was fleshed out and it needs to use specific types, especially because it has indexing implications
  • Different kinds of facets have different field needs
  • As we expand on content-types, we’ll need a more robust rich text system, which entities basically was
  • The term “entities” confused people

[dholms]

fixed #658