Typescript 4.2 fixed a return type for objects without a prototype.
This change throws an InvalidValueError for these, rather than the
TypeError that was previously thrown.
BREAKING CHANGE: this removes the test helpers, which were not well
tested and required the `async_hooks` module, and the PID file
helpers, which introduced a requirement on `fs` that could not be
easily polyfilled. This should make the library easier to use in
browsers and bundlers.