feat: implement expense tracker with CRUD operations and JSON storage

[AbelOsaretin]

Task Submission PR

Student Info:

• Registered Name: Abel Osaretin
• Week: 2
• Day: 5

Task Details:

• Task Name: Expense Tracker
• Task Description: A simple CLI expense tracker built in Rust. It supports adding, viewing, updating, and removing expenses, and persists data to a JSON file between runs.

Checklist Before Submitting:

• [*] My code follows the repository folder structure.
• [*] I have properly named my files and folders.
• [*] I have documented my code where necessary.
• I have tested my solution and ensured it works as expected.

Additional Comments:

If any, mention here.