use ./lexicons/ for default output (pluralize)

To keep it easy for multiple tools to work together, it would be great if the default output directory was "lexicons/" (plural), I think it is currently the singular "lexicon".

And also something like the norm in the atproto git repo, where 'app.bsky.feed.post' ends up at 'lexicons/app/bsky/feed/post.json', though that is more of a convention.