markdown: Restore fallback language highlighting for untagged code blocks (#63023)

Restores fallback highlights lost in
https://github.com/zed-industries/zed/pull/28217

Release Notes:

- N/A

README.md

@@ -46,4 +46,3 @@ Zed is developed by **Zed Industries, Inc.**, a for-profit company.
 If you’d like to financially support the project, you can do so via GitHub Sponsors.
 Sponsorships go directly to Zed Industries and are used as general company revenue.
 There are no perks or entitlements associated with sponsorship.
-