Use type 'string | Buffer' for process stdout/stderr

[robertbrignull]

The intention here is to avoid use of any when logging process stdout/stderr. This data comes to us in chunks which we want to call toString() on. We can do that with type safety if we introduce a asString method (very open to other names).

The only issues are the null and undefined values. In this I've gone with returning human-readable values since we're printing this to a log, but we could return the empty string instead. In practice I expect these values to never actually happen.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[koesie10]

Thanks, LGTM!