make `.stop()` flush

[eh-am]

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.