Customizing the development environment for Copilot coding agent

[AlekSi]

See https://docs.github.com/en/copilot/how-tos/agents/copilot-coding-agent/customizing-the-development-environment-for-copilot-coding-agent.

The Copilot agent does not utilize Dev Containers or Codespaces. Hopefully, that would change in the future.

In the meantime, we should create .github/workflows/copilot-setup-steps.yml with a single copilot-setup-steps job, as described in the link above.