HistoryCell is a trait (#2283)

refactors HistoryCell to be a trait instead of an enum. Also collapse
the many "degenerate" HistoryCell enums which were just a store of lines
into a single PlainHistoryCell type.

The goal here is to allow more ways of rendering history cells (e.g.
expanded/collapsed/"live"), and I expect we will return to more varied
types of HistoryCell as we develop this area.

codex-rs/tui/src/lib.rs

@@ -49,7 +49,6 @@ mod shimmer;
 mod slash_command;
 mod status_indicator_widget;
 mod streaming;
-mod text_block;
 mod text_formatting;
 mod tui;
 mod user_approval_widget;