feat: introduce CellWidget trait

[bolinfest]

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.