Fix unused step defs

[UL-ChrisGlew]

🤔 What's changed?

The 'Find Unused Step Definitions' context menu should no longer show step definitions decorated with the 'StepDefinition' attribute as unused when one or more is used with a Given/Then/When.

⚡️ What's your motivation?

Fix for #22.

🏷️ What kind of change is this?

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

♻️ Anything particular you want feedback on?

I've attempted to add a new unit test to test this. This involved adding some logic to correctly add a 'StepDefinition' attribute into the 'MockableDiscoveryService' - let me know if there's any better way of doing this.

📋 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
  • [ ] 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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[UL-ChrisGlew]

@gasparnagy I've updated the Changelog with the requested changes.

[gasparnagy]

@UL-ChrisGlew I added you to the group already, but you still deserve the "official" intro: 😎 Thx for the help!

Thank you for the contribution! According to our guidelines I have invited you to the Reqnroll contributors team. Congrats! 🎉 If you accept it, you will be able to make pull requests easier in the future.

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