Reference panel: display unhighlighted code in preview on timeout

Created by: olafurpg

Previously, the preview panel didn't show the actual code if syntax highlighting timed out. Now, the preview pane shows unhighlighted code instead.

Fixes #40835 (closed)

Test plan

Run sg start without the syntax-highlighter service to simulate a timeout for all files, and validate that the reference panel shows unhighlighted code

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.