Fix spawned wmic processes not exiting when using pidusage in packaged apps

[EthyMoney]

Q	A
Branch?	stable
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	n/a
License	MIT
Doc PR	n/a

With the recent addition of gwmi support on Windows, a spawn call was introduced to check for availability of wmic and switch to gwmi on failure.

This works fine by default, but introduces a new problem in cases where pidusage is being utilized in a packaged application such as with a tool like pkg or its actively maintained successor fork. In a packaged app, the spawned WMI processes don't get killed, which leads to a situation where each pidusage call creates a new process and they just infinitely stack up until the computer becomes unstable and/or runs out of memory.

I am someone who uses pidusage in packaged applications, so I found a simple solution to address this and created this PR. I modified lib/stats.js to now hold a reference to the spawned child wmic process, and I added a call to kill it in a finally block added to the try/catch. This maintains existing functionality while ensuring the child process is killed when this try/catch check is completed and works in both packaged and non-packaged environments.

For reference, this is what the task manager will look like when this situation occurs, with a new process forming on each pidusage call: