Refactoring/testing

- [x] prepare branch for unit testing (#20)
- [x] review phpcs integration
- [x] directly use phpcs classes for unit tests (quick approach)
- [ ] abstract literals from sniffs
    - [ ] standardize literal handling in sniffs
    - [ ] utilise an injectable approach for sniff literals
    - [ ] inject literals at runtime
    - [ ] simplify and review approach for internal discussion
- [ ] inject testable literals for tests
- [ ] drop actual literals from fixtures
- [ ] review test suite
- [ ] document approach for developers