[cpp] Prefer the longest step keyword

[musicinmybrain]

🤔 What's changed?

In C++, prefer the longest step keyword by sorting in descending order by length in bytes before matching.

⚡️ What's your motivation?

Fixes #400 for C++.

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

When combined with #414, I find that this fixes the C++ acceptance tests, which makes it a partial fix for #412.

Ideally, though, someone familiar with the gherkin language and the other implementations would also confirm that this fix looks right.

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes. N/A, fixes existing tests
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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