Show warnings for referenced packages that are out of date

When you pin package versions, there's nothing to remind you when package updates are available. If there was a warning shown at the end of each script execution, it would provide awareness to folks who aren't thinking about loading up their various packages' websites to check manually.

Similar to https://github.com/cake-build/cake/issues/2260, so I'll copy this part:

> Could a warning be too strong? Information might be a better fit except that it might fail in its purpose of drawing attention during a normal script execution. On the other hand, there's occasionally a CVE associated with a new update.
>
> Since the update check isn't mission-critical, it could be started asynchronously before compiling the script. If there is no network connectivity, the check can time out silently or be canceled silently when the script execution is finished.
>
> There should be a configuration setting for folks to opt out.

Start by reviewing the related discussion in issue #2260 and the package-version checking behavior described here. Define how warnings, asynchronous checking, silent failure or cancellation, and the opt-out configuration should work; the issue is complete when those choices are implemented and verified during script execution.

Written by the indexing model from the issue text.