add technical error to error message

feat: Enhance functionality, add tests, and improve docs

This commit introduces major improvements to the `gn` package, focusing on
functionality, developer experience, and robustness.

- Adds comprehensive documentation and examples to all public functions.
- Implements the standard `error` interface for the `gn.Error` type.
- Adds extensive unit tests, achieving full test coverage.
- Introduces a `Justfile` to streamline common development tasks like
  testing, linting, and building.
- Updates the `README.md` with detailed usage instructions and examples.
- Fixes bugs in message formatting and colorization.