Refactor: Add GeneratorWarnings as output parameter of UnitTestFeatureGenerator.

[clrudolphi]

🤔 What's changed?

The UnitTestFeatureGenerator's GenerateUnitTestFixture method now has an out parameter that is a collection of warnings that the generator has created. These would contain notices about problems that did not cause the generator to fail, but that the resulting Feature Test class may not be fully functional.

⚡️ What's your motivation?

In preparation for Cucumber Messages, this was added to allow the generator to signal that there was a problem in processing the Gherkin feature file that may impact Reqnroll's ability to emit Messages.

🏷️ What kind of change is this?

• :bank: Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

📋 Checklist:

• [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.
• [ ] Users should know about my change
  • [x] I have added an entry to the "[vNext]" section of the CHANGELOG, linking to this pull request & included my GitHub handle to the release contributors list.

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