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/bin/md-events.rs

@@ -0,0 +1,15 @@
+use std::io::Read;
+use std::io::{self};
+
+fn main() {
+    let mut input = String::new();
+    if let Err(err) = io::stdin().read_to_string(&mut input) {
+        eprintln!("failed to read stdin: {err}");
+        std::process::exit(1);
+    }
+
+    let parser = pulldown_cmark::Parser::new(&input);
+    for event in parser {
+        println!("{event:?}");
+    }
+}