Add getAllSources method

[davidjgoss]

🤔 What's changed?

Expose a getAllSources() method on the SupportCodeLibrary that returns an array of all source references from across parameter types, steps and hooks.

⚡️ What's your motivation?

A convenience for aggregating source URIs and detecting languages for generating snippets in cucumber/cucumber-node#36

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)

📋 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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