Disable definition lists in Markdown (#17648)

This PR disables definition list support in `pulldown_cmark`, as it is
has been causing a number of issues.

I opened an issue upstream with the panic we were seeing:
https://github.com/pulldown-cmark/pulldown-cmark/issues/957.

Release Notes:

- N/A

crates/markdown_preview/src/markdown_parser.rs

@@ -11,7 +11,9 @@ pub async fn parse_markdown(
     file_location_directory: Option<PathBuf>,
     language_registry: Option<Arc<LanguageRegistry>>,
 ) -> ParsedMarkdown {
-    let options = Options::all();
+    let mut options = Options::all();
+    options.remove(pulldown_cmark::Options::ENABLE_DEFINITION_LIST);
+
     let parser = Parser::new_ext(markdown_input, options);
     let parser = MarkdownParser::new(
         parser.into_offset_iter().collect(),