Rename option `workerStopTimeout` to `workerForceKillAfterTimeout`

[jagu-sayan]

I didn't understand that workerStopTimeout force kill the process when the time is reached after we call Worker.stop().
I was expecting same behavior than taskTimeout and I was confused between this 2 options.

I think it's more clear to call this option workerForceKillAfterTimeout like execa.