Implement insertAtEnd(element:E) function for a Doubly Linked List

[ashikka]

Taking reference from issue #48 , implement the insertAtEnd(element:E) function for a Doubly Linked List. This function will insert an element at the end of the Doubly Linked List.

First step

Search, read, and understand information about doubly-linked lists, and how they are implemented.

Second step

Open a pull request with the data structure implementation, and wait for comments from @code-sherpas/kollections-maintainers.

Minimum acceptance criteria

• The implementation must be well documented with proper Javadoc at Class and function levels.
• The implementation must be tested with automatic tests that cover all relevant cases and situations.
• The implementation must be generic.
• The implementation must have the expected time complexity according to Computer Science. Here you can find a very helpful cheat sheet.