Fix compatibility with being run in a child process

[OmgImAlexis]

This let's the process exit in the same way but still allows the streams to finish before child_process kills it.

Ref: nodejs/node#12921

[sindresorhus]

The linked issue is old and might not be the reality today. What exactly does this PR fix for you? Make sure you test with the latest Node.js 12 version.

[OmgImAlexis]

The issue is process.exit() doesn't wait for stdout to finish.

Ref: https://nodejs.org/api/process.html#process_process_exit_code

Calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

[OmgImAlexis]

@sindresorhus any chance this can be merged?