feat: Add support for environment variable configuration (#634)

[unseen2004]

 ### Summary
 This PR implements support for configuring Python Fire CLIs via environment variables, addressing issue #634. It allows users to pass configuration
  values securely (e.g., API keys) without exposing them in process lists.

 ### Key Features
 - **Naming Convention**: Parameters can be specified using `FIRE_<PARAM_NAME>` (e.g., `export FIRE_NAME=World`).
 - **Broad Support**: Works for positional arguments, keyword-only arguments, and `**kwargs`.
 - **Precedence**: Command-line arguments (flags/positional) correctly take precedence over environment variables.
 - **Type Safety**: Values from environment variables are parsed as Python literals (int, bool, list, etc.), matching standard Fire behavior.

### Changes
- **Logic**: Updated `fire/core.py` to include environment variable lookup during argument parsing.
- **Tests**: Added `fire/env_config_test.py` with 100% coverage for the new feature (including override and type parsing scenarios).
- **Docs**: Updated `docs/guide.md` and `docs/using-cli.md` with instructions and examples.

### Verification
- Verified by running the complete test suite (277 tests passed).
- Adheres to the Google Python Style Guide and project-specific conventions:
- PascalCase for methods.
- 2-space indentation.
- Single quotes for strings.