Process termination should also kill child processes

[gibson042]

eshost currently spawns child processes with detached: true: https://github.com/tc39/eshost/blob/3989a6bc659c1dc9e44236c10ea772a7f2d4e480/lib/ConsoleAgent.js#L100

As a result, killing an eshost process with e.g. SIGINT or SIGTERM leaves its children running. It should instead explicitly destroy() the child agents.