refactor: Replace StacSetValue implementation with a new structure stac_core (#363)

* refactor: Replace StacSetValue implementation with a new structure in stac_core

- Remove the old StacSetValue class and its associated files.
- Introduce a new StacSetValue widget in stac_core that manages application state through key-value pairs.
- Update exports to reflect the new StacSetValue parser and widget structure.

* fix: Handle null child in StacSetValue widget and convert to JSON for variable resolution

Author: divyanshub024