Refactor project directory structure for better organization

The current project structure lacks a clear organization, making it difficult to navigate and maintain. To address this issue, the directory structure has been reorganized.

Moved all .java files into a 'duke' package to encapsulate the main application logic and make it more modular.
Created 'task' sub-package to house Deadline, Event, Task, Todo
Also created 'util' sub-package to house Parser, Storage, StringToDateTime, TaskList, Ui, to provide a more structured approach to managing utility classes.
This reorganization follows Java's recommended package naming conventions, improving code readability and maintainability.

Additionally, this change enables better scalability for future feature additions or modifications.