Add New Calculator Function

Description

This PR adds a new operation to the calculator program, allowing users to calculate the exponent of two numbers.

Added a new function named exponent to the calculator.py file. This function takes two arguments, base and exponent, and returns the result of base raised to the power of exponent.
Updated the calculator function to include the new Exponent operation in the list of operations that the user can select.
Added a new print statement to display the option for the Exponent operation.
Updated the option number for the Quit operation to 6.
Added a new elif statement in the calculator function to handle the Exponent operation. This statement calls the exponent function with the user-provided numbers and prints the result.
Updated the if statement that checks if the user wants to quit the calculator to check if the choice is '6' instead of '5'.
Added a test case for the new exponent function to ensure its correctness.

Please review and merge this PR. Thank you!

Fixes #1.

To checkout this PR branch, run the following command in your terminal:

git checkout sweep/add-exponent-operation_2

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