replace tui_markdown with a custom markdown renderer (#3396)

Also, simplify the streaming behavior.

This fixes a number of display issues with streaming markdown, and paves
the way for better markdown features (e.g. customizable styles, syntax
highlighting, markdown-aware wrapping).

Not currently supported:
- footnotes
- tables
- reference-style links

codex-rs/tui/src/render/mod.rs

@@ -1,3 +1,2 @@
 pub mod highlight;
 pub mod line_utils;
-pub mod markdown_utils;