feat: introduce CellWidget trait (#1148)

The motivation behind this PR is to make it so a `HistoryCell` is more
like a `WidgetRef` that knows how to render itself into a `Rect` so that
it can be backed by something other than a `Vec<Line>`. Because a
`HistoryCell` is intended to appear in a scrollable list, we want to
ensure the stack of cells can be scrolled one `Line` at a time even if
the `HistoryCell` is not backed by a `Vec<Line>` itself.

To this end, we introduce the `CellWidget` trait whose key method is:

```
fn render_window(&self, first_visible_line: usize, area: Rect, buf: &mut Buffer);
```

The `first_visible_line` param is what differs from
`WidgetRef::render_ref()`, as a `CellWidget` needs to know the offset
into its "full view" at which it should start rendering.

The bookkeeping in `ConversationHistoryWidget` has been updated
accordingly to ensure each `CellWidget` in the history is rendered
appropriately.

codex-rs/tui/src/lib.rs

@@ -19,6 +19,7 @@ mod app;
 mod app_event;
 mod app_event_sender;
 mod bottom_pane;
+mod cell_widget;
 mod chatwidget;
 mod citation_regex;
 mod cli;
@@ -32,6 +33,7 @@ mod mouse_capture;
 mod scroll_event_helper;
 mod slash_command;
 mod status_indicator_widget;
+mod text_block;
 mod tui;
 mod user_approval_widget;