This PR adds the missing `process.reallyExit()` method to node's
`process` object.
Was [pinged on
twitter](https://twitter.com/biwanczuk/status/1663326659787862017)
regarding running the `fastify` test suite in node. They use `node-tap`
which has been around arguably the longest of the test frameworks and
relies on a couple of old APIs. They have `signal-exit` as a dependency
which in turn [makes use of
`process.reallyExit()`](8fa7fc9a9c/src/index.ts (L19)).
That function cannot be found anywhere in their documentation, but
exists at runtime. See
6a6b3c5402/lib/internal/bootstrap/node.js (L172)
This doesn't yet make `node-tap` work, but gets us one step closer.
