chore: add workflow for closing incomplete issues

[davidjgoss]

🤔 What's changed?

Use GitHub's recommended action to auto stale and close issues.

To begin with, just focusing on this flow for issues that we've labelled as "incomplete" (i.e. the requester hasn't provided enough information):

• After 3 weeks, mark stale and comment
• After 5 more days, close

⚡️ What's your motivation?

cucumber/common#2140

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing 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.

---

This text was originally generated from a template, then edited by hand. You can modify the template here.

[coveralls]

Coverage: 98.563%. Remained the same when pulling b8d651e on stale-workflows into ca4118d on main.