custom textarea (#1794)

This replaces tui-textarea with a custom textarea component.

Key differences:
1. wrapped lines
2. better unicode handling
3. uses the native terminal cursor

This should perhaps be spun out into its own separate crate at some
point, but for now it's convenient to have it in-tree.

codex-rs/tui/src/chatwidget.rs

@@ -509,6 +509,10 @@ impl ChatWidget<'_> {
         self.bottom_pane
             .set_token_usage(self.token_usage.clone(), self.config.model_context_window);
     }
+
+    pub fn cursor_pos(&self, area: Rect) -> Option<(u16, u16)> {
+        self.bottom_pane.cursor_pos(area)
+    }
 }
 
 impl WidgetRef for &ChatWidget<'_> {