Add `enableBashIntegration`, `enableFishIntegration`, and
`enableZshIntegration` options that evaluate `brew shellenv` to set up
Homebrew's environment and shell completions. This automates the
boilerplate that every nix-darwin Homebrew user currently writes manually.
All three shells use `interactiveShellInit`, consistent with direnv and
home-manager conventions. Fish additionally sets up completions paths in
the same hook.
