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/editor/src/hover_popover.rs

@@ -618,12 +618,12 @@ pub fn hover_markdown_style(window: &Window, cx: &App) -> MarkdownStyle {
         },
         syntax: cx.theme().syntax().clone(),
         selection_background_color: { cx.theme().players().local().selection },
-
         heading: StyleRefinement::default()
             .font_weight(FontWeight::BOLD)
             .text_base()
             .mt(rems(1.))
             .mb_0(),
+        ..Default::default()
     }
 }