markdown: Allow code blocks and tables to be horizontally scrollable (#25956)

This PR adds the ability for Markdown code blocks and tables to be made
horizontally scrollable.

This is a feature that the caller can opt in to.

Right now we're using it for the rendered Markdown in the Assistant 2
panel.

Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -134,6 +134,8 @@ impl ActiveThread {
             base_text_style: text_style,
             syntax: cx.theme().syntax().clone(),
             selection_background_color: cx.theme().players().local().selection,
+            code_block_overflow_x_scroll: true,
+            table_overflow_x_scroll: true,
             code_block: StyleRefinement {
                 margin: EdgesRefinement {
                     top: Some(Length::Definite(rems(0.).into())),