add julia to engine completion schema (#13163)

[cscheid]

This is a better fix for #13164

Fix the autocomplete suggestions for the engine field in VSCode by adding julia and markdown to the list of available options.

We don't add markdown on purpose, because markdown doesn't execute anything.

[posit-snyk-bot]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)