lookup: add on-headers

[bjohansebas]

Unfortunately, this module monkey-patches writeHead. It’s an important module in the Express ecosystem and probably in other key packages that need to use it as a hook for writeHead. Adding it to CITGM wouldn’t be a bad idea while we wait for a solution in Node.js core that would allow us to deprecate this module.

cc: @wesleytodd @UlisesGascon @blakeembrey @ctcpip

Checklist

• [x] npm test passes
• [x] tests are included
• [x] documentation is changed or added
• [x] contribution guidelines followed here

[bjohansebas]

I can't resolve the Prettier warning, it seems to be something outside of this PR.