stdlib: add direnv_leaf_dir function

[offbyone]

The purpose here is so that .envrc files that are invoked via
source_up can make use of the location of the outermost (leaf?)
.envrc, specifically its location.

For example, I am using this in a synced folder's top level .envrc:

db_root=$(realpath --relative-to "$HOME/Dropbox" "$(direnv_leaf_dir)")
if [[ $db_root == "." ]]; then
    db_root=Dropbox
fi
direnv_layout_dir="$HOME/.local/var/direnv/$db_root"

This lets me move my .direnv directories out of the sync, because the sync
directories are platform-specific but my Dropbox gets used on different CPU
architectures and OSs.

Docs

I didn't add stdlib docs because I didn't see the direnv_layout_dir function documented there either. I'm happy to add both.

Changelog

Same as with the docs; I can add this to the changelog, just tell me how you want it formatted and I'll update the PR.

Testing

I did not see any obvious place to add tests for the stdlib functions. If there is one, I'll happily add some.

[offbyone]

Would someone mind reviewing this?