feat: introduce the use of tui-markdown (#851)

This introduces the use of the `tui-markdown` crate to parse an
assistant message as Markdown and style it using ANSI for a better user
experience. As shown in the screenshot below, it has support for syntax
highlighting for _tagged_ fenced code blocks:

<img width="907" alt="image"
src="https://github.com/user-attachments/assets/900dc229-80bb-46e8-b1bb-efee4c70ba3c"
/>

That said, `tui-markdown` is not as configurable (or stylish!) as
https://www.npmjs.com/package/marked-terminal, which is what we use in
the TypeScript CLI. In particular:

* The styles are hardcoded and `tui_markdown::from_str()` does not take
any options whatsoever. It uses "bold white" for inline code style which
does not stand out as much as the yellow used by `marked-terminal`:


65402cbda7/tui-markdown/src/lib.rs (L464)

I asked Codex to take a first pass at this and it came up with:

https://github.com/joshka/tui-markdown/pull/80

* If a fenced code block is not tagged, then it does not get
highlighted. I would rather add some logic here:


65402cbda7/tui-markdown/src/lib.rs (L262)

that uses something like https://pypi.org/project/guesslang/ to examine
the value of `text` and try to use the appropriate syntax highlighter.

* When we have a fenced code block, we do not want to show the opening
and closing triple backticks in the output.

To unblock ourselves, we might want to bundle our own fork of
`tui-markdown` temporarily until we figure out what the shape of the API
should be and then try to upstream it.

codex-rs/tui/src/lib.rs

@@ -25,6 +25,7 @@ mod exec_command;
 mod git_warning_screen;
 mod history_cell;
 mod log_layer;
+mod markdown;
 mod scroll_event_helper;
 mod status_indicator_widget;
 mod tui;