Improve support for block directory

A `block.json` file is needed for the wordpress.org block directory. This should be added, either manually for each plugin, or as part of the bundling process.

For example: https://jhalabi.com/blog/gutenberg-block-json

https://github.com/WordPress/gutenberg/blob/trunk/docs/reference-guides/block-api/block-metadata.md
https://make.wordpress.org/plugins/2020/07/11/you-can-now-add-your-own-plugins-to-the-block-directory/
https://make.wordpress.org/core/2021/06/23/block-api-enhancements-in-wordpress-5-8/