make `.stop()` flush

This is going to be tricky.

When stopping the profiler, it only sets up a stop flag
https://github.com/pyroscope-io/pyroscope-nodejs/blob/09d3f4d1b77d8640181e871db0fa3d60919e4a7d/src/index.ts#L284
Which only affects the next profiling round
https://github.com/pyroscope-io/pyroscope-nodejs/blob/09d3f4d1b77d8640181e871db0fa3d60919e4a7d/src/index.ts#L257-L264

That means that it may take up to 10 seconds (the profiling round) to exit
https://github.com/pyroscope-io/pyroscope-nodejs/blob/09d3f4d1b77d8640181e871db0fa3d60919e4a7d/src/index.ts#L204

This behaviour is pretty clear when running tests, where the test runner hangs up to 10 seconds, and the last profile captured is full of `idle`, since the program it's not doing anything.