snippets: correctly escape backslashes in generated expressions

[davidjgoss]

🤔 What's changed?

When a generated cucumber expression contained a backslash (in order to escape, say, a parenthesis) the backslash would previously be treated as a noop escape in the context of a JavaScript string. Now we add another backslash to correctly escape it.

⚡️ What's your motivation?

Fixes #1324.

🏷️ What kind of change is this?

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

📋 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.
• 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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[coveralls]

Coverage increased (+0.003%) to 98.235% when pulling 641adf8 on fix/snippet-escapes into f26ce3e on main.