data_structures/linked_list: Add __str__() function (#3961)

* Adding __str__() function

* Removing white space

* Update data_structures/linked_list/__init__.py

Co-authored-by: xcodz-dot <71920621+xcodz-dot@users.noreply.github.com>

* Adding type hints

* Update __init__.py

* Update __init__.py

* Adding the changes requested

* Updating to fix pre-commit

* Updating __init__.py

* Updating __init__.py

Co-authored-by: xcodz-dot <71920621+xcodz-dot@users.noreply.github.com>