Add caching of parsed completion documentation markdown to reduce flicker when selecting (#31546)

Related to #31460 and #28635.

Release Notes:

- Fixed redraw delay of documentation from language server completions
and added caching to reduce flicker when using arrow keys to change
selection.

crates/markdown/examples/markdown.rs

@@ -67,14 +67,8 @@ struct MarkdownExample {
 
 impl MarkdownExample {
     pub fn new(text: SharedString, language_registry: Arc<LanguageRegistry>, cx: &mut App) -> Self {
-        let markdown = cx.new(|cx| {
-            Markdown::new(
-                text,
-                Some(language_registry),
-                Some("TypeScript".to_string()),
-                cx,
-            )
-        });
+        let markdown = cx
+            .new(|cx| Markdown::new(text, Some(language_registry), Some("TypeScript".into()), cx));
         Self { markdown }
     }
 }