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/chatwidget/snapshots/codex_tui__chatwidget__tests__chatwidget_markdown_code_blocks_vt100_snapshot.snap

@@ -0,0 +1,18 @@
+---
+source: tui/src/chatwidget/tests.rs
+expression: visual
+---
+>     -- Indented code block (4 spaces)
+      SELECT *
+      FROM "users"
+      WHERE "email" LIKE '%@example.com';
+
+  ```sh
+  printf 'fenced within fenced\n'
+  ```
+
+  {
+    // comment allowed in jsonc
+    "path": "C:\\Program Files\\App",
+    "regex": "^foo.*(bar)?$"
+  }