Use `ProcessHandle` instead of shell script to check for parent process

[datho7561]

See #1025

[fbricon]

Code LGTM, but I haven't tested it. @datho7561 have you?

[datho7561]

I made sure it still runs as normal, but I haven't tried killing the parent process manually yet, I should do that...

[angelozerr]

What about binary? Is it working?

[datho7561]

I made sure that lemminx starts up properly, then closed all VS Code abnormally (killall -9 code), and ensured that eventually the lemminx process stopped. I did this for the java version and the binary on Linux.