Add valueStringifier for reporting

[pimartin]

This PR adds an optional valueStringifier property in the DetailedSpec that allows the user to customize the way the received value is displayed in the errors report.

This allows secrets to be redacted to prevent them from appearing in the logs for example.

If not provided, it defaults to using JSON.stringify, thus preserving backward compatibility.

[lostfictions]

thanks for the contribution (and sorry to be slow to get around to a review). this seems like a reasonable feature to me.

it looks like the documentation you added is positioned incorrectly, and prettier needs to be run. could you also add a few tests for this? feel free to simply add them to the end of parse-env.test.ts.

what's the expected behaviour if valueStringifier throws?

[lostfictions]

thanks for adding documentation! looks like you inserted this in between the description of defaults and its example, so it should be moved downwards. could you also add the use-case of redacting values that you wrote up in the docstring?

[pimartin]

Thanks for your comments, I will update the PR in accordance.

[lostfictions]

As a heads up, I've been thinking about this a bit this past week alongside how to fix #2 -- I may introduce a more low-level API for printing values that allows more fine-grained control over printing (including colorization), as well as a "compatibility" entrypoint whose default stringifier performs no colorization at all (to unblock usage on edge runtimes and in the browser). So things may shift around a bit -- feel free to hold off for now, I'll keep this use case in mind!