Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
* zellij: adds options to integrate with zsh, bash and fish shells
* zellij: add tests for shell integration options
* zellij: eval setup auto start for fish integration
* zellij: use interactiveShellInit for fish integration
* zellij: fixes format issues
* zellij: enable shell integrations by default
* zellij: compresses shell integration test cases
* zellij: removes the disabled shell integration tests
* zellij: formats tests
