Improve `baseUrl` handling for relative URLs

Remove unnecessary baseUrl storage from generated classes and streamline
relative URL resolution to only occur during JSON-LD decoding.

Changes:

- Remove baseUrl property and getter from generated vocabulary classes
- Remove baseUrl option from class constructors
- Add explicit baseUrlVar parameter to scalar type decoder methods
- Use optional chaining (options?.baseUrl) for safer URL resolution
- Maintain relative URL support without storing baseUrl in instances

This improves upon PR #443's approach by avoiding unnecessary property
storage while still resolving relative URLs when baseUrl is provided
during fromJsonLd() calls.