fix: restrict semantic tokens and completions to gherkin files

[zohnannor]

🤔 What's changed?

Added a languageId check to the connection.languages.semanticTokens.on and connection.onCompletion handlers in CucumberLanguageServer.ts; if we are not in a 'gherkin' file, do nothing.

⚡️ What's your motivation?

Fixes cucumber/vscode#305

🏷️ What kind of change is this?

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

♻️ Anything particular you want feedback on?

N/A

📋 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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