Update mockup diff visualization

[conorsexton]

This PR updates the visualization of mockup data in the Destination panel (#59)

Instead of a <textarea> element, we’re leveraging the React Diff Viewer library.

Now, ResponseComponent cards hide the modified data by default. Users can click “Show diff” to view a GitHub style diff of any changes.

Known issues

• Clicking a line number will throw an error: onLineNumberClick is not a function since we haven’t defined a function to process line clicks
• Large diffs don’t automatically collapse like with GitHub. This is a limitation of the library itself, but it still seemed like a good solution based on how straightforward it was to implement. Given more time, we could consider switching to a more feature-rich library like react-diff-view