Generate metadata file for each version

[OrKoN]

This allows clients to find all metadata for a given version without the need to fetch the (larger) file with all versions.

[mathiasbynens]

This is a much more scalable approach that still enables people to get the download URLs from the API instead of hardcoding them. Thanks!

Some TODOs before we land this:

• Can we move this into generate-extra-json.mjs?
• Let’s document this in the README.md as well.

[OrKoN]

Done, PTAL.