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/snapshots/codex_tui__markdown_render__markdown_render_tests__markdown_render_complex_snapshot.snap

@@ -0,0 +1,62 @@
+---
+source: tui/src/markdown_render_tests.rs
+expression: rendered
+---
+# H1: Markdown Streaming Test
+
+Intro paragraph with bold text, italic text, and inline code x=1.
+Combined bold-italic both and escaped asterisks *literal*.
+Auto-link: https://example.com (https://example.com) and reference link [ref][r1].
+Link with title: hover me (https://example.com) and mailto mailto:test@example.com (mailto:test@example.com).
+Image: alt text
+
+> Blockquote level 1
+> 
+> > Blockquote level 2 with inline code
+
+- Unordered list item 1
+    - Nested bullet with italics inner
+- Unordered list item 2 with strikethrough
+
+1. Ordered item one
+2. Ordered item two with sublist:
+    1. Alt-numbered subitem
+
+- [ ] Task: unchecked
+- [x] Task: checked with link home (https://example.org)
+
+———
+
+Table below (alignment test):
+| Left | Center | Right |
+|:-----|:------:|------:|
+| a    |   b    |     c |
+Inline HTML: <sup>sup</sup> and <sub>sub</sub>.
+HTML block:
+<div style="border:1px solid #ccc;padding:2px">inline block</div>
+Escapes: \_underscores\_, backslash \\, ticks ``code with `backtick` inside``.
+Emoji shortcodes: :sparkles: :tada: (if supported).
+Hard break test (line ends with two spaces)  
+Next line should be close to previous.
+Footnote reference here[^1] and another[^longnote].
+Horizontal rule with asterisks:
+***
+Fenced code block (JSON):
+```json
+{ "a": 1, "b": [true, false] }
+```
+Fenced code with tildes and triple backticks inside:
+~~~markdown
+To close ``` you need tildes.
+~~~
+Indented code block:
+    for i in range(3): print(i)
+Definition-like list:
+Term
+: Definition with `code`.
+Character entities: &amp; &lt; &gt; &quot; &#39;
+[^1]: This is the first footnote.
+[^longnote]: A longer footnote with a link to [Rust](https://www.rust-lang.org/).
+Escaped pipe in text: a \| b \| c.
+URL with parentheses: [link](https://example.com/path_(with)_parens).
+[r1]: https://example.com/ref "Reference link title"