Document/configure supported node version(s)

[askoufis]

I was going to make a PR to update the flat-cache dependency to the latest major, but given that it dropped support for Node <16, I figured it was worth consulting this package's support node version. However, I didn't find one documented anywhere.

Given that the package still supports webpack 4, is the intention to support whatever minimum version webpack 4 supports (Node 6 or something)? If not, I think it would be worth declaring supported node versions via the package.json#engines field, or similar.