[feature request] support `sync` mode

[aprilandjan]

Hi, thanks for your helpful module! This is a feature request. Can we have sync mode so I can correctly block my other code executions while using? For example:

const treeKill = require('tree-kill').sync;

console.log('try kill', pid);
treeKill(pid);
console.log(pid, 'killed')

[vladpuz]

I switched to this:

https://github.com/nktnet1/kill-sync