Downgrade severity level of undefined steps to warning

[kieran-ryan]

🤔 What's changed?

• Downgraded the severity level of diagnostics for 'undefined steps' from error to warning

⚡️ What's your motivation?

The Cucumber Language Service uses Language Server Protocol (LSP) diagnostic severity levels. These enums differ from the same diagnostic severity levels represented by the Monaco editor API.

Assumption is that while integrating the Language Service with the Monaco editor, it was observed that errors were being classified as 'hint' and warnings as 'info'; and thus a mitigation was to cast all severity levels to Monaco 'error' severity levels.

This change rectifies the issue by mapping intended severity levels of the language service (LSP levels) to those of the Monaco editor API.

Improves distinction between gherkin syntax errors and undefined step warnings.

🏷️ 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] Users should know about my change
  • [x] I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.