Bibclean

- Fix packaging of default configuration
- Add entry-point for CIs: `bibclean-check` which exits with code `0` if the provided file is correctly formatted, else `1` if violations are found and `2` if the configuration is invalid or an issue was encountered.

Consolidate the initial release:
- Add a configuration module:
- Includes a default configuration with the required fields and the fields to keep for each entry type.
- It will host the configuration loader for TOML entries.
- Non-supported entry types retain all their fields, re-ordered alphabetically.
- All entries, including the unsupported types, are re-ordered alphabetically.
- Error message now explicitly mention the problematic entries.

Initial release covering the ``article`` entries of a BibTex file.