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/markdown/examples/markdown_as_child.rs

@@ -84,6 +84,7 @@ pub fn main() {
                         selection
                     },
                     heading: Default::default(),
+                    ..Default::default()
                 };
                 let markdown = cx.new(|cx| {
                     Markdown::new(MARKDOWN_EXAMPLE.into(), markdown_style, None, None, cx)