Add scrollbars to markdown preview and syntax tree view (#38183)

Closes https://github.com/zed-industries/zed/issues/38141

This PR adds default scrollbars to the markdown preview and syntax tree
view.

Release Notes:

- Added scrollbars to the markdown preview and syntax tree view.

crates/markdown_preview/src/markdown_preview_view.rs

@@ -13,7 +13,7 @@ use gpui::{
 use language::LanguageRegistry;
 use settings::Settings;
 use theme::ThemeSettings;
-use ui::prelude::*;
+use ui::{WithScrollbar, prelude::*};
 use workspace::item::{Item, ItemHandle};
 use workspace::{Pane, Workspace};
 
@@ -481,7 +481,7 @@ impl Item for MarkdownPreviewView {
 }
 
 impl Render for MarkdownPreviewView {
-    fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         let buffer_size = ThemeSettings::get_global(cx).buffer_font_size(cx);
         let buffer_line_height = ThemeSettings::get_global(cx).buffer_line_height;
 
@@ -598,5 +598,6 @@ impl Render for MarkdownPreviewView {
                     .size_full(),
                 )
             }))
+            .vertical_scrollbar_for(self.list_state.clone(), window, cx)
     }
 }