syntax-highlight bash lines (#3142)

i'm not yet convinced i have the best heuristics for what to highlight,
but this feels like a useful step towards something a bit easier to
read, esp. when the model is producing large commands.

<img width="669" height="589" alt="Screenshot 2025-09-03 at 8 21 56 PM"
src="https://github.com/user-attachments/assets/b9cbcc43-80e8-4d41-93c8-daa74b84b331"
/>

also a fairly significant refactor of our line wrapping logic.

codex-rs/tui/src/lib.rs

@@ -58,6 +58,7 @@ mod streaming;
 mod text_formatting;
 mod tui;
 mod user_approval_widget;
+mod wrapping;
 
 // Internal vt100-based replay tests live as a separate source file to keep them
 // close to the widget code. Include them in unit tests.