fix: gracefully close open sockets

[matt-smarsh]

When attempting a graceful shutdown, if a page is open with a HMR websocket, it will prevent shutting down. Use the onClose hook to shut down the dev server.

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

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[galvez]

LGTM, thank you.