0.18.0 release

New features:
- mk: exclude .venv from flake8 checks
- init: add pybuild-plugin-pyproject to Build-Depends
- release: detect git platform for correct tag URL format
- init: add GitHub Actions workflow template
- init: add --platform option for GitHub/GitLab selection
- ci: add GitHub Actions reusable workflow
- docs: add GitHub Actions settings for Pages deployment
- pyproject: update maintainers
- pkg: add PKGBUILD for Arch Linux packaging
- mk: add pkg target for Arch Linux package building
- release: update PKGBUILD version during release
- init: add Dockerfile.ci-archlinux template
- gitlab-ci: add Arch Linux package building
- github-actions: add Arch Linux package building
- pkg: add Arch Linux repository support
- init: skip flit init if pyproject.toml exists
- init: add --force flag to overwrite existing files
- gitlab-ci-pipeline: ci-image-archlinux: run on MRs