sum_tree: Replace rayon with futures (#41586)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Kate <kate@zed.dev>

crates/markdown_preview/src/markdown_parser.rs

@@ -779,7 +779,7 @@ impl<'a> MarkdownParser<'a> {
 
         let highlights = if let Some(language) = &language {
             if let Some(registry) = &self.language_registry {
-                let rope: language::Rope = code.as_str().into();
+                let rope = language::Rope::from_str_small(code.as_str());
                 registry
                     .language_for_name_or_extension(language)
                     .await