Output the URLs of the issues/PRs that were found

[skedwards88]

This PR:

• Updates the action to set the output to the list of issue URLs found
• Explicitly skips some sections of the code if the project input was not provided. (I think this was already happening since the projectBoard input isn't required, but this makes it more explicit in the code.)
• Upgrades the actions/core package to deal with a deprecation issue
• Changes the @zeit/ncc dependency (no longer maintained) to @vercel/ncc to deal with a build issue following the actions core upgrade
• Upgrades to node 16 since node 12 is deprecated
• Updates the README to reflect the output

Now that the list of URLs is returned, people can use this action to find team pings and then take whatever downstream action that they want. For example, they can add the returned items to a Project v2 and update project fields for the item.

For an example of a workflow running this updated action, see https://github.com/github/docs-content/actions/runs/7071424385/job/19249135402