Provide a way to write machine-readable output to stdout for pack

[lengau]

What needs to get done

As an opt-in (probably CLI argument, but maybe also available by environment variable), allow the user to get a JSON output of the packed objects for pack rather than the human-readable final message.

Document how this is done so applications can do the same thing for their own commands.

Why it needs to get done

Makes it easier to use craft apps within scripts.