Base64 encode string literals in JsonFormatter

[badeball]

🤔 What's changed?

This fixes #2260.

This ensures that all embeddings in JSON reports are base64-encoded. This is also how cucumber-json-formatter interprets IDENTITY strings.

⚡️ What's your motivation?

In the JSON output, it's currently impossible to differentiate between strings that were attached as literals vs. those that were base64-encoded to begin with.

🏷️ What kind of change is this?

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

📋 Checklist:

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

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[coveralls]

coverage: 98.474%. remained the same when pulling ee9a1b85d983ae8e8481ad4687a48a32f8398543 on base64-encode-literals into 188d64445a27fd7bf546bf3b0a6c0163667b96ca on main.