Highlight differences when changing features

[ericglau]

When adding or removing a selected feature, it may not be obvious what parts of the resulting code has changed. This is especially the case for Cairo because there is a lot of code that requires scrolling to view.

Enhance the UI so that differences are visibly highlighted when changing a feature.

For example, if the user starts with:

then enables Mintable like the below:

, this should cause the following sections to be highlighted:

import "@openzeppelin/contracts/access/Ownable.sol";

and

    function mint(address to, uint256 amount) public onlyOwner {
        _mint(to, amount);
    }