Phonecodes

Added
- Support for converting between the Buckeye corpus phonetic alphabet and IPA

Changed
- Made language explicitly optional for conversions between ARPABET and X-SAMPA
- Minor updates to README and CONTRIBUTIONS

Added
- Modern python packaging, including pyproject.toml
- Contribution guidelines
- Changelog with sematic versioning
- Usage examples in README
- Unit tests using pytest and triggered by Github Action on pull request
- Checks and linting with Ruff

Changed
- Ordering of language and corpus code in phonecodes.pronlex.read
- Removed duplicated dictionary keys in phonecodes.phonecode_tables