🚀 Example: Add one for the `diff` property

[JoshuaKGoldberg]

Example Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues, but none matched my issue.
• I want to provide a PR to resolve this

Overview

From https://mochajs.org/#diffs:

Mocha supports the err.expected and err.actual properties of any thrown AssertionErrors from an assertion library. Mocha will attempt to display the difference between what was expected, and what the assertion actually saw. Here’s an example of a “string” diff using --inline-diffs:

No examples demonstrate this diffing functionality right now.

Suggested Solution

How about we add an example that can show a nice string diff?

Additional Info

This would be nice for mochajs/mocha#5348: a PR that bumps Mocha's diff dependency.