Refactor calculate function to use safer method for mathematical evaluation

Description

This PR updates the implementation of the calculate function in calculator.js to improve security by avoiding the use of the eval function. Instead of eval, the Function function is now used to evaluate mathematical expressions more safely.

Changes made

Replaced the use of eval with the Function function to evaluate mathematical expressions.
The result is converted to a string before being displayed in the result-display.
Errors are handled more explicitly, displaying 'Error' in case of an exception.

Context

This change addresses security related to the use of eval. By making this change, we improved the robustness and security of the code, while maintaining the expected functionality of the calculator.

Functionality Check

Testing has been performed to ensure that calculator functionality is not adversely affected and that changing the calculate function works as expected.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

LauraNicole449 / calculator