feat: add unit tests.

[siavashyj]

Rename project and implement unit tests

Description

This pull request renames the project from "playlist-downloader" to "youtube-playlist-downloader" and implements comprehensive unit tests for the main functionality.

Changes

• Renamed the main project directory from playlist-downloader to youtube-playlist-downloader
• Updated .pre-commit-config.yaml with the latest versions of black, isort, and flake8
• Added a new tests directory with __init__.py and test_main.py
• Implemented unit tests for all main functions: get_authenticated_service, get_playlists, get_playlist_items, and main
• Updated README.md with instructions on how to run unit tests
• Adjusted project structure to better accommodate testing and follow Python package naming conventions

Testing

• Added pytest as a development dependency
• Implemented mock objects for YouTube API interactions to enable testing without actual API calls
• Covered various scenarios including different combinations of playlist retrieval options

How to test

1. Clone the updated repository
2. Install dependencies: poetry install
3. Run the tests: poetry run pytest

Additional Notes

• The new test suite provides better coverage and should help maintain code quality as the project evolves

Please review the changes and let me know if any further modifications are needed.

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️