Improves the test coverage of the project and adds examples for coverage reports.
- Increases the test coverage of the `cmd/trix` package from 67.7% to 78.1%.
- Increases the test coverage of the `pkg/crypt` package from 96.2% to 98.7%.
- Adds tests for the `examples` to ensure they run without errors.
- Adds a new example that demonstrates how to generate and interpret a coverage report.
