markdown: Don't retain MarkdownStyle in favor of using MarkdownElement directly (#28255)

This PR removes the retained `MarkdownStyle` on the `Markdown` entity in
favor of using the `MarkdownElement` directly and passing the
`MarkdownStyle` to it.

This makes it so switching themes will be reflected live in the code
block styles.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/git_ui/src/git_panel.rs

@@ -3927,9 +3927,9 @@ impl GitPanelMessageTooltip {
                     }),
                 };
 
-                this.update_in(cx, |this: &mut GitPanelMessageTooltip, window, cx| {
+                this.update(cx, |this: &mut GitPanelMessageTooltip, cx| {
                     this.commit_tooltip = Some(cx.new(move |cx| {
-                        CommitTooltip::new(commit_details, repository, workspace, window, cx)
+                        CommitTooltip::new(commit_details, repository, workspace, cx)
                     }));
                     cx.notify();
                 })