Add new calculator function for exponent operation

[ghost]

Description

This PR adds a new function for the exponent operation in the calculator program. It also updates the calculator function to include a new option for the exponent operation and adds error handling for any ValueError that may occur during the calculation.

Summary of Changes

• Added a new function named 'exponent' in calculator.py that takes two arguments, a and b, and returns the result of a raised to the power of b using Python's built-in pow function.
• Updated the calculator function in calculator.py to include a new option for the exponent operation. The user can now select this option and input the numbers for the exponent calculation.
• Added error handling in the exponent function to catch any ValueError that may be thrown by the pow function. If an error occurs, a user-friendly error message is printed.
• Updated the README.md file to include documentation for the new exponent operation.

Please review and merge this PR. Thank you!

Fixes #246.

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[lukejagg]

In the exponent function, can you make the function error "Exponent error: " instead of "error"