Enable cross-workspace definition lookup

[LeoAnders]

Summary

• allow DocumentContentProvider to search configured sibling workspace folders before falling back to the server
• expose a new objectscript.export.searchOtherWorkspaceFolders setting and document the cross-workspace Go to Definition workflow
• add multi-root fixtures and integration tests that ensure the definition provider resolves local files across workspace folders

Testing

• npm run lint
• npm test (fails: unable to download VS Code in the test harness environment)