Add shell completion scripts

I added shell completions scripts for bash, fish and zsh. Those scripts directly came from https://github.com/microsoft/vcpkg folder scripts/. They can be downloaded by adding https://github.com/microsoft/vcpkg/ to sources, but since they are small and haven't been changed for years, I directly copied them into this repository. Cloning the entire microsoft/vcpkg repository can be time consuming, especially for just three files.