terminateProcess() should not use cp.spawnSync() or cp.execFileSync()

[bpasero]

Looks like terminateProcess is being used from the renderer and has 2 occurrences of cp.execFileSync() and cp.spawnSync() that should use the async variant instead. It should be possible to just return a promise from this method.

Feel free to move forward if you are not the owner of this method, I just went by git history.terminateProcess