Add new packages field to corral.json

[SeanTAllen]

The packages field is a new experimental field that will list all
packages that are part of a bundle that should be included for distribution.

At the moment, entries must be added by hand, this commit assures that
any that exist are not removed when corral manipulates the corral.json file.

Eventually, if this feature holds, we will have commands to add and remove
packages as well as a command to prepare for release. Releases, rather than
being a "repo over here with tag" can be hosted on services like GitHub packages
or cloudsmith and would only include the bare minumum needed for the package.

Initially, this field is being used to generate documentation that works across
library projects. Existing exploratory work for that is being done in the
ponylang/library-documentation-action repository.

[ponylang-main]

Hi @SeanTAllen,

The changelog - added label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 167.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.