Set environment variables needed for bash, zsh completion in opam env

[WardBrian]

Would close #6427

This updates opam env to set XDG_DATA_DIRS (from the XDG base directory specification) to include the current switch's share root, and FPATH to include %{share}%/zsh/site-functions when the shell is zsh

The former works with bash-completion, which looks in the XDG_DATA_DIRS entries for bash-completion/completions sub-directories, while the later sets up zsh's built-in completion mechanisms to look in this location.

Questions for reviewers --

1. I currently only modify FPATH on zsh, but I think it would be safe to do it generically (the only other shell that seems to assign meaning to the FPATH variable is ksh, which opam doesn't support)
2. Assuming we want to keep FPATH zsh-specific, is it worth threading through a shell variable all the way from the CLI to where we compute these updates, or is it fine to keep the guess_shell_compat call?