Split the click groups and commands into separate modules

[hotenov]

TO DO

To simplify maintenance and future changes, it's need to implement more complex code structure.

As help references:

• https://click.palletsprojects.com/en/latest/quickstart/#nesting-commands
• https://stackoverflow.com/questions/34643620/how-can-i-split-my-click-commands-each-with-a-set-of-sub-commands-into-multipl

[hotenov]

Now package structure is

playlist_along
│   cli.py
│   playlist.py
│   py.typed
│   _utils.py
│   __init__.py
│   __main__.py
│
├───commands
│   │   convert.py
│   │   display.py
│   │   __init__.py