Compare actual/expected screenshots in acceptance tests

[mattwynne]

🤔 What's changed?

• Modified acceptance tests to save a screenshot of every rendered example, and use pixelmatch to compare with an expected one.

⚡️ What's your motivation?

I think this package needs better tests so that we can confidently deal with edge-cases like #120.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

♻️ Anything particular you want feedback on?

We could also save off the rendered HTML - and compare that. I feel the diffs would be too hard to read and so not very useful. Might help for debugging though?

Or maybe it would be better to make specific assertions about the rendered DOM?

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct

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