T23a: AI chat, messages

- messages: yours in a quiet bubble, the assistant's as prose beside its mark,
  system notes, errors; avatars, headers with model and time, footers
- MessageList over gpui's bottom-aligned list: drawn near the view, held at the
  newest, counting arrivals while away; a way back down; day separators
- streaming text and markdown reveal toward the text as it grows, with a caret;
  a blinking cursor before the first words
- CodeBlock with language, copy, run, apply, download and folding;
  MarkdownRenderer::code lets messages draw fences with it
- pictures, grids, files, voice notes, video over host frames, link previews
- quotes, actions, an editor that takes focus, versions, regenerate, stop and
  continue
- errors with retry, a rate limit counting down, feedback, thinking blocks
- gallery: the AI Chat page with scripted states

src/lib.rs

@@ -2,6 +2,7 @@ mod assets;
 
 pub mod buttons;
 pub mod charts;
+pub mod chat;
 pub mod collab;
 pub mod data_display;
 pub mod debug;