Implement Entry, Income, Expense, and Budget classes

[goldmonkey777]

Completed all iterations:

• ✅ Iteration 1: Entry class with constructor and getFormattedAmount()
• ✅ Iteration 2: Income class extending Entry with type="income"
• ✅ Iteration 3: Expense class extending Entry with type="expense", paid property, and overridden getFormattedAmount()
• ✅ Iteration 4: Budget class with addEntry() and getCurrentBalance() methods
• ✅ Bonus: getFormattedEntries() method using forEach()

All tests should pass.