Create workflow to automerge `sourcery-ai[bot]` PRs

[brendanator]

• Create GitHub workflow called merge-sourcery-ai-bot-pr.yml
• Trigger on pull request
• Set env vars
  • GH_TOKEN: secrets.USER_TOKEN
• Add permissions needed: content
• Only if the author is sourcery-ai[bot] merge the PR using gh pr merge --squash --auto --delete-branch --repo <repo> <pr_number>