feat: add dev container Features for modular toolchains
Add Features (gcc, llvm, cpp-tools, conan, python-venv, iwyu, ydotool, scala, apt-mirror, shell-aliases) targeting debian:stable-slim, each with tests, plus a combined C++ scenario, consumer templates and a GitLab CI pipeline that tests on merge requests and publishes on tags.
Source-built Features purge only the build tools they installed so they do not break other Features in the same image.