* EMBED-1484- add extension-sdk and extension-sdk-react packages to mono repo

1. extension-sdk package added
2. extension-sdk-react package added
3. tests for both packages work
4. both packages lint correctly
5. extension-api-explorer extension updated to utilize monorepo extension packages
6. hackathon extension updated to utilize monorepo extension packages
7. added lint script that only displays errors

Co-authored-by: John Kaster <kaster@google.com>