- Updated reference validation to handle various identifier types including DOI, ISBN, ISSN, URN, and Handle.
- Improved regex patterns for DOI and Handle validation to correctly extract and validate identifiers from URLs.
- Added asynchronous checks to verify the existence of DOI and Handle URLs.
- Added asynchronous checks to verify the existence of ISBNs
- Included detailed comments explaining the regex patterns and validation logic.
- Adjusted the validation logic to handle any URL prefix for Handle identifiers.
- Ensured that the Handle format `handle/20.500.12854/36478` is correctly validated.
- Updated the CI workflow to trigger on push and pull request events.
