show a deprecation warning on when using `util.inherit`

[jimmywarting]

b/c inherit is discouraged could you give the ppl a warning for using it so ppl will update?

Usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. @nodejs

[ljharb]

In what node version was this warning added?

(either way, node-based tests should be emitting this warning regardless)

[goto-bus-stop]

It's not deprecated, just legacy, and extremely stable, and it works just fine. a runtime warning would be far too invasive (node.js also doesn't have one).