Add GitHub actions: CodeQL, Pylint, Stale

[yantosca]

This PR adds YAML configuration files to the .github/workflows folder so that certain GitHub Actions can be triggered when commits are pushed. We have activated:

• CodeQL security scan (pushes to main, dev)

• Pylint (code linter, with Python 3.9, 3.10, 3.11)

• Stale (tags issues and PRs as stale)

[github-advanced-security[bot]]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.